Preprint: Detection of Anti-H5 Antibodies in People with Exposure to Wild Birds in Northern Canada

 

#19,185

Almost exactly 2 years ago - 3 months after the discovery of HPAI H5N1 in U.S. dairy cows - the CDC released the results of their study on Population Immunity to A(H5N1) clade 2.3.3.4b Viruses.  They reported:

CDC analyzed sera (blood) collected from people of all ages in all 10 HHS regions. Blood samples were collected during the 2022-2023 and 2021-2022 flu seasons. These samples were challenged with H5N1 virus to see whether there was an antibody reaction. 

Data from this study suggest that there is extremely low to no population immunity to clade 2.3.4.4b A(H5N1) viruses in the United States.  Antibody levels remained low regardless of whether or not the participants had gotten a seasonal flu vaccination, meaning that seasonal flu vaccination did not produce antibodies to A(H5N1) viruses.

This was in the United States, of course, where exposure to HPAI H5N1 has historically been very low. Over the years we've looked at a number of other seroprevalence studies - particularly in regions with known outbreaks - which have produced slightly different results.

• In 2004 (see The Thailand Serological Study) 322 poultry farmers (in provinces where H5N1 had been detected) were tested. Researchers found that "no poultry workers had microneutralization titers >80, whereas 7 (2%) had lower titers that did not meet the WHO definition for seropositivity".
• In May of 2009 (see Cambodian Study Finds Rare Asymptomatic H5N1 Infections) we saw a study published in the Journal of Infectious Diseases on more than 600 members of a Cambodian village where 2 human H5N1 cases were detected in 2006. Antibody titers showed that only 1% (7 of 674) of the villagers tested had contracted, and fought off, the H5N1 virus. A figure much lower than many had expected.

• In 2012, in H5N1 Seroprevalence Among Jiangsu Province Poultry Workers, we saw a study that found across three locations tested (Gaochun, Jianhu and Gaoyou counties) the percentage of workers testing positive ranged from zero (Gaochun) to 5.38% (95%CI, 2.19%–10.78%) in Gaoyou.

• More recently, in 2024's MMWR: Serologic Evidence of Recent Infection with HPAI A(H5) Virus Among Dairy Workers, in a relatively small sampling (n=115) 7% (n=8)` . . . of exposed dairy farm workers in Michigan and Colorado had serologic evidence of infection with HPAI A(H5).'

Not unexpectedly, people with direct exposure to infected poultry or livestock tend to have higher detectable levels of HPAI H5 antibodies than the general population. There are a few caveats to serological tests, including:

• They only pick up past infections ( >1-3 wks), and are not designed to detect current infections
• Detectable levels of influenza antibodies can wane over time, meaning older or milder exposures might no longer be detectable
• Exposure to some subtypes can sometimes produce cross-neutralizing antibodies to another (see EID Journal: A(H5N1) NA Inhibition Antibodies in Healthy Adults after Exposure to Influenza A(H1N1)pdm09).

In other words - while incredibly useful for retrospective investigations - there has always been some degree of ambiguity regarding the results of seroprevalence testing.

All of which brings us to a preprint, published last week, which looks as a small sampling (n=65) of blood samples collected from people (mostly hunters) with exposure to wild birds in Northern Manitoba, Canada. 

The authors reports a ` 7.4% (n=5/68) anti-H5 seroreactivity rate among hunters in Northern Canada'. Some of these hunters had exposure to small mammals as well.  

The authors list some of the limitations to their study, stating:

The relatively small sample size of the study provides limited power for statistical analyses, and given that the study was conducted in a small region of Manitoba, future larger-scale studies are needed to evaluate how commonly people with exposure to wild birds may be infected by H5N1. 

Unfortunately, the durability of the antibody response to H5N1 is unknown, therefore we can only conclude that the five seroreactive individuals have been exposed to the virus at some during their lifetimes, although the durability of antibodies against avian influenza in human populations is unknown. Whether these individuals experienced symptomatic or asymptomatic infections, and whether they would be protected from subsequent re-infection, is also unknown.

I've reproduced the summary and a few excerpts below, but you'll want to follow the link to read this (not-yet peer-reviewed) report in its entirety.  I'll have a bit more after the break. 

Detection of Anti-H5 Antibodies in People with Exposure to Wild Birds in Northern Canada
Hannah L. Wallace, Morgan Hiebert, Mikayla Hunter, Megan Halbrook, Ryan J. Harrigan, Isaac I. Bogoch, Anne W. Rimoin, Souradet Shaw, Linda Larcombe, Pamela H. Orr, Jason Kindrachuk
doi: https://doi.org/10.64898/2026.05.24.26353994         

Posted May 26, 2026.

        PDF 

Summary

Using a commercially available H5 serology assay, we identified a 7.4% (n=5/68) anti-H5 seroreactivity rate among hunters in Northern Canada. All participants reported close contact with wild birds. 

The Study 

In August 2025 (three months after the 2025 spring goose hunting season), 68 dried blood spot (DBS) samples with matched questionnaires (Appendix 1) were collected from a northern community in Manitoba, Canada. Recruitment was focused on those > 18 years old who were involved in the hunting of birds and/or preparing them (plucking, butchering, food preparation) for consumption.

Some of the participants were also involved in hunting, trapping and handling of mammals (Table 1). Participants were recruited through social media posts and community posters. Participation was voluntary, and included both local community members as well as individuals who were visiting the community for the upcoming fall goose hunting season. Participants were compensated for their involvement in the study. This study was approved by the University of Manitoba Health Research Ethics Board under study HS26941 (H2025-151). 

        (SNIP)

Conclusions

This work indicates that people living and/or hunting in northern Manitoba, Canada have been exposed to H5Nx viruses. Knowledge and concerns about H5N1 among study participants was low, indicating a need for more robust and accessible public health information. Future work including sampling a larger number of individuals and performing longitudinal follow-up to determine how antibody levels change over time would be of significant value. Overall, this work adds to the growing body of literature that aims to understand the true extent of spillovers of avian-origin influenza viruses into humans. 

        (Continue . . . ) 

This isn't the first time we've looked at the potential increased HPAI risk to hunters (and to hunting dogs). Last February we saw Several States Warn On Contact With Wild Birds/Mammals, and the USDA has warned of the risks for several years.

Of course, hunting isn't the only high risk activity.

• In 2024's Mixed Messaging On HPAI Food Safety - we looked at the risks from the slaughtering of live birds and preparation of raw poultry; especially from birds raised at home or purchased from live markets.

• There are scores of LBMs (live bird markets) in the United States (see USDA Report 9 More Live Bird Markets Infected With HPAI H5), and millions of families raise, and slaughter, their own backyard poultry. 

Sadly, the public appears largely unconvinced of the threat from HPAI (see Two Surveys (UK & U.S.) Illustrating The Public's Lack of Concern Over Avian Flu), which almost certainly provides the virus with more opportunities to spread and evolve.  

QJM: Avian Influenza in Humans: Virology, Transmission, and Clinical Priorities

Credit WHO

#19,184

While there are no signs that avian influenza is spreading in an efficient or sustained manner between humans, there are concerns that some spillover infections are going unrecognized, and each instance provides with virus with another opportunity to adapt to a human host.

Retrospective antibody testing has shown that some infections are either mild, or subclinical (see JAMA Open: Asymptomatic Human Infections With Avian Influenza A(H5N1) Virus Confirmed by Molecular and Serologic Testing).

Although HPAI H5N1 (clade 2.3.4.4b) is currently viewed as the most worrisome avian flu virus, there are many others, including other H5 subclades (2.3.2.1c or 2.3.2.1a), other H5Nx subtypes, H7 viruses, H9N2, H3N8, and H10Nx. 

We've also seen reports of atypical presentation (both mild and severe) with avian influenza, along with difficulties in testing some patients, even in a modern hospital setting.

Because of this, we've seen many instances where patients have been hospitalized for days or even weeks before their avian flu infection was finally confirmed.  A few examples:

• In the fall of 2024, a Missouri man was hospitalized for a week - then released - only to be notified that he had tested positive for H5N1

• In June of 2025, we saw a Statement on a Fatal H5N2 Infection In Mexico City, which we would eventually learn, was only detected 2 weeks after the patient had died. 

• Last April, in Eurosurveillance, we looked at an imported fever/cough case in Italy who initially tested negative for influenza A/B, RSV & COVID, but after a more invasive BAL (Bronchoalveolar lavage), was identified as having H9N2 on the 6th day of his hospitalization. 

• And 3 weeks ago, the MMWR report on the fatal H5N5 case in Washington State last year repeatedly tested negative for influenza/COVID during the first 6 days of his hospitalization.

While avian flu normally presents as a respiratory infection, we've also seen cases where the symptoms were primarily gastrointestinal, neurological, or subclinical. 

In April of 2025 we saw a preliminary report on a neuroinvasive infection in an 8-y.o. girl (see Vietnam: Ho Chi Minh DOH Reports A Rare H5N1 Encephalitis Case In a Child). While her throat and nose swabs tested negative for influenza A, H5N1 was detected in the patient's cerebrospinal fluid.

As noted by infectious experts, this is a rare case in which the A/H5N1 avian influenza virus damages the central nervous system and does not attack the respiratory tract.

All of which brings us to a narrative review - published this week in the QJM - which argues that avian flu is no longer just a `poultry exposure risk', as its many variants continue to expand both their geographic and (avian & mammalian) host ranges around the globe. 

The opportunities for spillover into humans have increased markedly over the past few years, which makes its important for clinicians to raise their index of suspicion - particularly during times of known outbreaks - even when dealing with atypical presentations or negative test results. 

While this review is `avian flu specific', much of it applies to swine and other novel flu viruses as well. Due to its length, and technical nature, I've only posted some excerpts. Follow the link to read it in its entirety. 

Avian Influenza in Humans: Virology, Transmission, and Clinical Priorities

Nitin Gupta , Anna Smielewska , Jan Felix Drexler , Casandra Bulescu , Marta Mora-Rillo , Aleksandra Barac , Pikka Jokelainen , François-Xavier Lescure , Martin P Grobusch , Sotirios Tsiodras ... Show more
QJM: An International Journal of Medicine, hcag138, https://doi.org/10.1093/qjmed/hcag138
Published:29 May 2026 Article history

PDF
 
Abstract

Avian influenza continues to evolve as a zoonotic threat with important implications for clinical practice and global health preparedness. Sustained circulation in wild birds, repeated spillover into poultry, and an increasing number of infections across diverse mammalian hosts have reshaped exposure pathways and broadened the risk landscape for human infection. For clinicians, this evolving ecology translates into atypical presentations and increased diagnostic uncertainty. Recent global activity has been characterised by widespread animal outbreaks and the emergence of new transmission interfaces, including occupational exposures and livestock-associated events. Human infections remain largely zoonotic and geographically heterogeneous, with patterns influenced by surveillance intensity, exposure context, and healthcare access.

We synthesise current evidence on the virology, transmission, global epidemiology, clinical manifestations, diagnosis, treatment, and prevention of avian influenza in humans. We highlight evolving mammalian adaptation and changing risk interfaces that complicate risk assessment. Improved clinician awareness, early diagnosis, and integrated One Health surveillance remain central to strengthening preparedness for future influenza threats.

(SNIP)

Clinical Presentation and Diagnosis of Avian Influenza in Humans

Human infection with avian influenza viruses spans a broad clinical spectrum, ranging from asymptomatic infection to severe, rapidly progressive multisystem disease (Table 1) [2,56]. Asymptomatic or subclinical infection has increasingly been recognised through serosurveys and occupational surveillance programmes, indicating that mild infections are likely underdetected outside targeted screening settings [57,58]. 

In a serosurvey of bovine veterinary practitioners in the United States during the 2024 H5 outbreak in dairy cattle, antibodies indicating recent infection were detected in 3 of 150 participants (2%), none of whom reported respiratory or influenza-like symptoms [58]. Similar findings from studies among poultry workers have demonstrated measurable seropositivity in individuals without recognised illness [57]. 

These observations indicate that reliance on clinically apparent cases may underestimate the true burden of zoonotic infections and lead to overestimation of case fatality rates based solely on detected severe cases. 

(SNIP)

Neurological complications, including encephalopathy, encephalitis, seizures, convulsions, and altered consciousness, have been reported in severe H5N1 infection and may occasionally dominate the clinical picture, even in the absence of prominent respiratory disease [69,70].

Human case reports describe acute encephalitis with detectable H5N1 RNA in cerebrospinal fluid, including a child with minimal respiratory symptoms who developed encephalitis complicated by obstructive hydrocephalus, supporting direct central nervous system involvement [71]. Other reported manifestations include diffuse encephalitis, coma, and rapidly progressive neurological deterioration in the setting of severe systemic infection.

Neuropathological and postmortem studies have demonstrated viral antigen and RNA in neurons and glial cells across multiple brain regions, supporting direct neuroinvasion rather than solely secondary inflammatory injury [69]. Experimental studies in ferrets further support this neurotropic potential, showing that H5N1 can invade the brain, often via the olfactory pathway, and may be associated with encephalitis, vasculitis, hemorrhagic lesions, and even subclinical but persistent brain injury [72].

       (SNIP) 

Conclusion

Avian influenza is ecologically entrenched, globally mobile, and increasingly multi-host. The expansion of H5Nx viruses into previously unafflicted regions, diverse mammalian species, combined with persistent circulation in wild birds and poultry, underscores that the conditions for emergence are continuously present. Apparent epidemiological lulls may reflect surveillance artefacts rather than reduced risk. 

In a world where birds migrate, viruses reassort, and agricultural systems are intensified, preparedness, not prediction, remains the most reliable defence. Strengthening One Health surveillance, ensuring rapid clinical recognition, and maintaining flexible medical countermeasures are essential to prevent avian influenza from becoming the next global pandemic. For clinicians worldwide, recognising avian influenza as an evolving zoonotic interface rather than a rare exotic infection will be central to early detection and response.

        (Continue . . . )

California: LA County HAN (Health Alert Network) Advisory on Upcoming World Cup

Credit Wikipedia

#19,183

As we discussed at some length last January (see Public Health Ontario: Hazard Identification and Risk Assessment (HIRA) For the FIFA World Cup 2026 Games in Toronto), this summer 16 cities across 3 North American countries (Canada, Mexico, U.S) will host the 2026 world cup, with over 5 million fans expected to travel to the various venues.

As with all mass gathering events (see Potential Impact of Hajj 2025 & Other Mass Gathering Events On Public Health) there are always public health concerns. 

While some of the more exotic diseases (avian flu, MERS-CoV, Ebola, etc.)  are often the first threats that come to mind, most infectious illnesses acquired during these mass gathering/migration events are far more common; seasonal flu, COVID,  pneumonia, measles, meningococcal disease, mpox, norovirus, and vector borne infections (Zika, CHKV, Dengue, Malaria, Yellow Fever, etc.) (see CDC's Traveler's Health Saudi Arabia).

Not all public health threats will be from infectious diseases; as increased heat-related illnesses, accidents, substance use, and overdoses are also often associated with these types of events. 

Accordingly, this week the LA County Department of Public Health released the following HAN Advisory for health care providers on the upcoming World Cup festivities. 

Similar advisories will no doubt be issued by other World Cup venues, as local  hospitals, EMS units, and public health agencies gear up to provide essential services during these events. 

Taiwan Amends Travel Restrictions Due to Ebola Threat

International Air flights - Credit Wikipedia

#19,182

Two days ago, in ECDC Ebola Update & Individual Country Responses From Canada, U.S. & Taiwan, we looked at recently imposed travel restrictions and protocols due to the central African Ebola outbreak here in the U.S., in Canada, and in Taiwan. 

Although the there are long-standing international agreements not to restrict travel and trade during disease outbreaks (see 2005 IHR agreement), as Tip O'Neill famously noted; `. . . all politics is local'. 

We saw this adage in action during the opening days of COVID, when much of the advice contained in the WHO's 2019 91-page Influenza NPI guidance document was quickly abandoned by member nations; particularly their recommendations against border closures and travel restrictions. 

High-minded ideals (even when backed by science) hold little sway when public officials are forced explain to their constituents why they didn't even try to prevent the entry of a pandemic or epidemic virus.  

Citing similar moves by both Canada and the United States, today Taiwan's government has amended their previously announced policy, and have suspended the issuance of entry visas from the DRC and Uganda (with 4 exceptions) for the next 90 days. 

While not a huge story in and of itself, this is a reminder of how fluid the Ebola situation is - how concerned governments really are -  and how quickly policies may be amended or changed. 

As we discussed two days ago, there is no `one-size-fits-all' solution to the Ebola threat, and so we are likely to see a wide range of responses, with varying degrees of success.  

Stay tuned.

The full translated announcement follows.

In response to the escalating Ebola outbreak, starting from 00:00 on June 2, 2026, entry into the Democratic Republic of Congo and Uganda will be suspended for 90 days, except for four categories of individuals.

Release Date: 2026-05-29

The Centers for Disease Control (CDC) stated today (May 29) that, according to data released by the WHO as of May 27, 2026, the COVID-19 outbreak continues to expand in the Democratic Republic of Congo (DRC) and Uganda, primarily affecting Ituri, North Kivu, and South Kivu provinces within the DRC. The DRC has reported a cumulative total of 906 cases (223 deaths) and 125 confirmed cases (17 deaths). Uganda has reported a cumulative total of 7 confirmed cases to date, including 1 death. Based on the assessment of the Taiwan Centers for Disease Control (CDC), the Ebola outbreaks in the Democratic Republic of Congo and Uganda are likely to continue to escalate. 

In addition to strengthening cross-airport joint prevention and control measures and enhancing port monitoring and interception mechanisms, Taiwan, in order to further reduce the risk of imported cases and referencing practices in the United States and Canada, has jointly discussed with the Ministry of Foreign Affairs and the Bureau of Consular Affairs, the National Immigration Agency of the Ministry of the Interior, and the Civil Aeronautics Administration of the Ministry of Transportation and Communications, and will implement new border epidemic prevention and control measures as follows:

I. Starting from 00:00 on June 2, 2026, the issuance of visas to residents of the Democratic Republic of Congo and Uganda will be suspended; those already issued visas will have their entry temporarily suspended for 90 days. However, the following four categories of individuals will be excluded:

(I) Students who have already obtained admission permission from Taiwan

; (II) Diplomatic and official duties

; (III) Spouses of Taiwanese citizens who are not Taiwanese citizens and their minor children

; (IV) Emergency or humanitarian assistance: such as attending funerals or visiting seriously ill relatives.

II. Taiwanese citizens who have traveled to epidemic areas within 21 days prior to entry, holders of valid Taiwanese residence permits, and those permitted to enter Taiwan are still subject to the measures announced by the Centers for Disease Control and Prevention (CDC) on May 27. They must proactively report to the airport quarantine station upon arrival in Taiwan, where quarantine personnel will issue an "Inbound Passenger Self-Health Management Notice." They must conduct self-health management for 21 days after entry, and follow the requirements of the notice to report their health status. If symptoms appear, they should immediately call the epidemic prevention hotline 1922 for assistance from health authorities.

The CDC explained that the aforementioned control measures will be adjusted in a timely manner based on the latest international epidemic situation and the epidemic prevention risks at Taiwan's borders. The CDC reiterated that the travel epidemic recommendation level for the Democratic Republic of Congo (DRC) and Uganda is Level 3 "Warning," urging the public to avoid all non-essential travel to these countries.

The Taiwan Centers for Disease Control (CDC) reiterates that, to strengthen border quarantine, in addition to enhancing public awareness through airport multimedia electronic billboards, scrolling displays, and signage, it has been making in-flight announcements on all international flights arriving in Taiwan since May 27th. Passengers who have traveled to the Democratic Republic of Congo or Uganda within the past 21 days are urged to proactively report to the quarantine station upon arrival in Taiwan for TOCC and health assessment. Please cooperate with the following quarantine measures:

1. Passengers assessed as having suspected Ebola virus infection symptoms (fever, headache, muscle pain, nausea, vomiting, abdominal pain, diarrhea, or bleeding, etc.) will be immediately transported by ambulance to a contracted hospital for examination, and local health authorities will be coordinated in their prevention and control efforts.

2. Asymptomatic passengers will be issued a "Notice of Self-Health Management for Passengers with Travel History to Ebola-Epidemic Areas." Upon arrival, please cooperate with 21 days of self-health management, keep your phone accessible for contact tracing by health authorities, take your temperature twice daily (morning and evening), and report your health status to the "Public Proactive E-Reporting System." If you experience any of the above symptoms, please call the epidemic prevention hotline 1922 immediately for assistance from the Health Bureau to seek medical attention. Those who do not cooperate with the above measures will be penalized in accordance with the Infectious Disease Prevention and Control Act.

WHO DON Update: Hantavirus outbreak linked to cruise ship travel, Multi-locations

 

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Although the hantavirus outbreak among the passengers and crew of the m/v Hondius has slowed, 3 new cases have been reported over the past 2 weeks, and the isolation, quarantine, and/or monitoring of exposed individuals continues. 

While additional hantavirus cases are still possible, a much larger threat has since emerged in Central Africa with the rapidly expanding Bundibugyo virus outbreak in the DRC and Uganda.  

Yesterday the WHO released their first formal DON update on the Andes Hantavirus outbreak since May 13th, which adds the 3 latest cases, and reiterates that the global risk level from this outbreak remains low.

Due to its length, I've only posted some excerpts. Follow the link to read it in its entirety.  

Hantavirus outbreak linked to cruise ship travel, Multi-locations

28 May 2026

Situation at a glance

This is the fourth Disease Outbreak News report on the Andes hantavirus outbreak linked to cruise ship travel, following the notification to the World Health Organization (WHO) on 2 May 2026 of severe respiratory illness cases aboard M/V Hondius, a cruise ship. Since the last DON was published on 13 May, three additional confirmed cases were reported, from Canada, the Netherlands, and Spain. The previously reported inconclusive case from the United States of America was subsequently determined to be negative following further laboratory testing and has been removed from the total case count. All cases to date have been passengers or crew members on the ship. 

As of 27 May, a total of 13 cases, including three deaths, have been reported (case fatality ratio 23%). Eleven cases have been laboratory-confirmed for Andes virus (ANDV) infection, and two are probable cases. Given the long incubation period of up to six weeks, it is not unexpected that cases continue to be reported until the end of the six weeks since last exposure. Through the International Health Regulations (2005) (IHR) channels, National IHR Focal Points (NFPs) have all been informed and are supporting international contact tracing and monitoring efforts. WHO has assessed the risk posed by this event to the global population as low and will continue to monitor the epidemiological situation and update the risk assessment as needed.

(SNIP)

High-risk contacts are being quarantined and monitored by local health authorities either in their respective countries or in the ship’s flag country, the Netherlands, or third countries (Table 1). As of 22 May 2026, more than 600 contacts, including 53% high-risk and 47% low-risk contacts, have been identified across 32 countries, territories and areas, and are either under close monitoring or self-monitoring in line with the updated guidance on management of contacts of Andes virus (ANDV) cases from the MV Hondius cruise ship published on 17 May.

WHO risk assessment

WHO continues to assess the risk for passengers and crew who were onboard the cruise ship as moderate, as individuals exposed prior to the implementation of control measures may still develop illness during the incubation period and should therefore be closely monitored.

The risk at the global level is assessed as low for the following reasons:

• Andes virus has demonstrated limited human-to-human transmission in previous outbreaks, typically occurring among close contacts and within household settings, generally requiring prolonged close exposure. Transmission can be contained through early detection, isolation of cases, clinical management, and contact management. However, the ship environment presented an increased risk due to close living quarters, shared indoor spaces, prolonged exposure, and frequent interpersonal interactions, all of which likely facilitated transmission.
• Human Pulmonary Syndrome caused by hantaviruses in the Americas, including Andes virus, can have a high case fatality ratio, reaching 40-50%, particularly among elderly individuals and those with co-morbidities. The average age of passengers on board the ship was 65 years old.
• Investigations on the travel history and potential exposures of the first case in the Southern Cone subregion of the Americas are ongoing and suggest possible exposure to rodents during recreational activities. Viral sequencing analyses are also ongoing and are comparing the ANDV strain associated with this outbreak with strains circulating in Argentina and Chile, where the disease is enzootic. The preliminary sequencing analysis for the cases indicates a high degree of genetic similarity amongst sequenced cases —showing no more than one single nucleotide polymorphisms difference per individual – which strongly indicates that the outbreak likely arose from a single zoonotic spillover event, or from a very small number of closely related spillover events.[1]
• Additional cases may occur among individuals exposed before implementation of containment measures. However, the current response, including quarantine for those who have left the ship and rapid isolation of any new suspect cases and the monitoring of contacts, is expected to limit the risk of further spread.
• As there is no specific antiviral treatment for HPS, suspected cases require prompt transfer to an adequately equipped emergency department or intensive care unit, where available, for close monitoring and supportive management to improve chances of recovery. Consequently, for remote areas, rapid transfer to a well-resourced healthcare facility is required, which may be challenging under the current conditions.

For the general public, including people not exposed on board the ship or through close contact with a confirmed case, the overall probability of infection remains low. Current evidence indicates that human-to-human transmission occurs through close and prolonged contact, and can be effectively limited through early detection, isolation of cases, and contact tracing.

   

        (Continue . . . )

JAMA Network Open: Long COVID Persistence and Surveillance Gaps Across 58 US Hospitals

#19,180

For a variety of economic, political, and societal reasons most of the world's nations have moved towards `normalizing' COVID infection; treating it more as if it were the `flu' or the common `cold'. 

Testing outside of the hospital environment is now uncommon, and ICU admissions and deaths are no longer published by 90% of the world's nations.

Although COVID deaths have dropped, the evidence continues to show that COVID infections - and particularly repeated infections - can still take a considerable toll on human health. 

While `Long COVID' and PASC (Postacute Sequelae of COVID) are now officially recognized conditions, its presentation is often `messy'; with multiple diffuse - and sometimes conflicting - symptoms.  

A few studies of note include:

The Lancet: Long COVID and Risk of Incident Cardiovascular Disease

EID Journal: Thrombotic Events and Stroke in the Year After COVID-19 or Other Acute Respiratory Infection

Nature: Long-term Cardiovascular Outcomes of COVID-19

AHA: COVID-19 May Trigger New-Onset High Blood Pressure

NIH: Study Shows SARS-CoV-2 Infects Coronary Arteries, Increases Plaque Inflammation

EHJ: Accelerated Vascular Ageing After COVID-19 Infection: The CARTESIAN Study

While we've seen estimates of up to 1 in 5 adults experiencing some form of PASC, other studies have shown a much lower incidence; sometimes in the single digits. We've also seen studies that suggest - since the arrival of Omicron in late 2021 - the incidence and/or severity of `Long COVID' has dropped. 

But nearly all of these reports rely heavily on the medical coding systems, which adopted a `Post-COVID syndrome' ICD-10 code (U09.9) in late 2021, but which is only used at the discretion of the treating physician. 

Some clinicians may avoid coding for PASC because it is largely a diagnosis of exclusion, and they may want to rule out other causes first. Others may consider it too broad, and prefer to code specific complaints like fatigue, dyspnea, or cognition problems. 

As a result, when studies are based on EHR (Electronic Health Records) coding, they may miss many probable PASC cases. 

In order to try to remove - or at least narrow - this blind spot, researchers created an AI tool that searched electronic health records for patterns of symptoms and diagnoses consistent with PASC, even though their chart may not have been coded as such.

There are limitations to this type of approach, as it relies heavily on the quality and quantity of the EHR documentation, and so it may have missed some PASC cases. At the same time, temporal association does not establish causation, and so these numbers should be taken with a grain of salt. 

Still, it strongly suggests that the actual burden of PASC is considerably higher than the EHR coding reflects, and that the incidence of Long COVID was still increasing in 2024, two years after the shift to Omicron. 

The full study is well worth reading in its entirety.  I've posted the Abstract and summary below. 

Original Investigation
Infectious Diseases
Long COVID Persistence and Surveillance Gaps Across 58 US Hospitals
Jiazi Tian, MSc1; Alaleh Azhir, MD, MSc1,2; Matthew Decaro, MSc3 et al

JAMA Netw Open
Published Online: May 27, 2026
2026;9;(5):e2614909. doi:10.1001/jamanetworkopen.2026.14909

Key Points

Question What is the true burden of chronic disease following COVID-19, and why does current surveillance fail to capture it?

Findings In this cohort study of 457 950 patients with COVID-19 across 58 hospitals, validated computable phenotyping identified postacute sequelae of SARS-CoV-2 infection in 16.28% of cases, 2-fold higher than diagnostic code–based surveillance. Of identified manifestations, 89.31% represented chronic conditions, with prevalence increasing through mid-2024.

Meaning These findings suggest that approximately 1 in 6 patients with COVID-19 develops postacute sequelae, predominantly chronic conditions currently invisible to surveillance systems, representing an accumulating rather than resolving health care burden.

Abstract

Importance Surveillance of postacute sequelae of SARS-CoV-2 infection (PASC) depends on diagnostic coding systems that capture fewer than one-half of affected individuals, rendering millions invisible to health systems and policymakers.

Objective To quantify the gap between true PASC burden and diagnostic code–based estimates, determine the proportion representing chronic disease, and characterize organ system heterogeneity and temporal trends across diverse populations.

Design, Setting, and Participants This retrospective cohort study used electronic health record data from 58 hospitals and affiliated clinics in 4 US regions, from 2017 to 2025. Adults (aged ≥18 years) with laboratory-confirmed SARS-CoV-2 infection or a COVID-19 diagnosis code were included. A custom artificial intelligence algorithm, the Precision Phenotyping for Research Cohorts (P2RC), was implemented using federated infrastructure.

Exposure Laboratory-confirmed SARS-CoV-2 infection or COVID-19 diagnosis code.

Main Outcomes and Measures The primary outcomes were PASC prevalence, the proportion classified as chronic conditions, organ system distribution, and temporal trends from 2020 to 2024. χ2 Tests were used to assess organ system heterogeneity across regions, and negative binomial regression was used to model quarterly temporal trends, yielding incidence rate ratios (IRRs) with 95% CIs.

Results In this cohort study of 457 950 COVID-19 cases (mean age, 52.05 years; 275 107 [60.07%] female), the P2RC algorithm identified 74 560 PASC cases (16.28% overall; 28 585 [18.58%] in New England, 978 [19.55%] in Southeast Texas, 10 534 [22.69%] in Southern California, and 34 463 [13.64%] in Western Pennsylvania), more than 2-fold higher than the proportion identified by code-based surveillance (<7%). Of 883 International Statistical Classification of Diseases, Tenth Revision, Clinical Modification codes associated with PASC, 594 (67.27%) represented chronic or potentially chronic conditions. Of 74 560 patients with PASC, 66 587 (89.31%) developed chronic conditions requiring ongoing clinical management; this represents 14.54% of the total number of 457 950 patients with COVID-19. Substantial organ system heterogeneity was observed (χ2 = 2504.73; P < .001): New England demonstrated thyroid-predominant endocrine patterns, while Southeast Texas, Southern California, and Western Pennsylvania showed metabolic-predominant profiles. Negative binomial regression revealed increasing PASC prevalence through mid-2024 (IRR per quarter, 1.01 [95% CI, 1.00-1.01; P < .001] in New England; 1.00 [95% CI, 1.00-1.01; P < .001] in Southern California; and 1.02 [95% CI, 1.01-1.02; P < .001] in Western Pennsylvania), indicating an accumulating rather than resolving burden.

Conclusions and Relevance In this cohort study, approximately 1 in 6 patients with COVID-19 developed PASC, and 89.31% of these patients had at least 1 chronic condition. Current diagnostic coding captured fewer than one-half of the cases, obscuring a substantial chronic disease burden. The persistently increasing prevalence through 2024 indicated an accumulating health care burden requiring investment in surveillance infrastructure and integrated care pathways.

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