Nature Health: York University Study on Containment scenarios for H5N1

Credit NIAID

#19,100

A number of countries have procured quantities of a pre-pandemic H5N1 vaccine for human use (United States (4.8 million doses), the UK (5 million doses), Canada  (500,000 doses) Japan (10 million doses), and the EU (initially 664,000 doses but increased last year to 27+ million doses). 

With the exception of a special release of 20,000 doses of H5N1 vaccine to Finland following their 2023 fur-farm epizootic (see Finland: MOH Announcement On Avian Flu Vaccine Availability For People At High Risk), these vaccines remain locked away, as nations debate under what circumstances, and who would receive them.

Since it is assumed that it would require two doses - given 30 days apart - to convey (likely limited) immunity, only a small percentage of the population can be vaccinated in the opening months of a pandemic.  

It is widely expected that it could take 6 months or longer before any substantial quantity of vaccine would be available to the general public (see Maggie Fox's SCI AM - A Bird Flu Vaccine Might Come Too Late to Save Us from H5N1).

We've looked at various proposals for deploying existing pre-pandemic vaccines (see CMAJ: Avian Influenza and Use of the H5N1 Vaccine to Prevent Zoonotic Infection in Canada), but no strong consensus has emerged.  

However, two groups are frequently mentioned.

• People who handle live avian influenza A(H5N1) virus in laboratory settings  
• People with ongoing contact with known infected birds or other known infected animals or their environments

Exactly how and when to pull that trigger, and how receptive people will be to taking the vaccine, are big unknowns.  

This week, Nature has published a study which considers some options on how best to contain a spillover of avian flu from poultry to humans (presumably farmers). 

 It is, alas, behind a paywall, but we do have a press release from York University to fill in the gaps. First the link to the study/abstract.

Containment scenarios for post-spillover transmission chains of avian influenza H5N1 from poultry to humans

Affan Shoukat, Chad R. Wells, Gergely Röst, Alison P. Galvani, Meagan C. Fitzpatrick & Seyed M. Moghadas
Nature Health (2026)Cite this article

Inspired by the Canadian teen who contracted H5N1 in the fall of 2024 without an obvious exposure risk (see NEJM: Critical Illness in an Adolescent with Influenza A(H5N1) Virus Infection) the authors modeled several approaches to containing a spillover to humans at a B.C. poultry farm.

Isolation of infected individuals is the first obvious step, but not every case will be symptomatic (see JAMA Open: Asymptomatic Human Infections With Avian Influenza A(H5N1) Virus Confirmed by Molecular and Serologic Testing). 

Application of a `Tamiflu Blanket' may prevent some infections, but data on PEP (Post-Exposure Prophylaxis) is limited (see J.I.D.: Antivirals for Novel Influenza A Virus Infections), and antiviral resistant strains have been reported (see Emerg. Microbes & Inf: Oseltamivir Resistant H5N1 (Genotype D1.1) found On 8 Canadian Poultry Farms).

Reactive vaccination - performed after the outbreak has occurred - is hampered by the 30-40 days it takes to build immunity, whereas pre-emptive vaccination of high risk individuals (i.e. famers) negates that concern.

The authors write:

. . .  reactive vaccination has very limited additional benefits outside of self-isolation, but pre-emptive vaccination adds substantial additional benefits on top of self-isolation. 

I've reproduced some excerpts from the press release below. Follow the link to read it in its entirety.  I'll have a bit more after the break. 

Researchers model how to contain Avian flu H5N1 in case of human-to-human transmission

By Sandra McLean
March 27, 2026

At this point, Avian flu H5N1 is thought to have very limited ability to transmit between humans, but a recent case in British Columbia with an unknown source of transmission has piqued the curiosity and concern of scientists, including York University Professor Seyed Moghadas.

Did this lone case come about through transmission from an animal or another person, and if it was via human transmission, what methods will control its spread in the human population? Director of York’s Agent-Based Modelling Laboratory in the Centre of Excellence in AI for Public Health Advancement, Moghadas and a group of researchers used modelling to understand the best spread control measures should human-to-human transmission become possible.

“The idea was, let's evaluate some of the interventions that we usually implement at the very earliest stage of a disease outbreak or emerging disease, which we know very little about,” he says.

For the research, "Containment Scenarios for Post-Spillover Transmission Chains of Avian Influenza H5N1 from Poultry to Humans,” published today in Nature Health, various scenarios from isolation to vaccination before or after a spillover event were modelled. It is one of only a few studies that have explicitly modelled outbreak dynamics following spillover into humans or the effectiveness of public health interventions in early and highly uncertain phases of virus development.

(SNIP)

The researchers used Abbottsford, B.C. as the location as it is a highly dense poultry farming area. The starting point is after a spillover has happened. 

“If a human became infected, how do we block this single individual to trigger a large outbreak? Or if the infection is going on between humans, can we block these chains and to what degree we can block them?” asks Moghadas. “What is the effectiveness of either self-isolation of symptomatic cases or vaccination of farmers or vaccination of farmers and their household members?”

Even with mitigation measures, someone in the farmer’s family could potentially be infected by the farmer and then transmit it to someone in the community.

The team evaluated two different types of vaccination strategies. One was reactive, which means that you trigger a vaccination program after a case has been identified somewhere. The second strategy was pre-emptive – individuals, such as farmers, are vaccinated before any case is identified.

What they found is that reactive vaccination has very limited additional benefits outside of self-isolation, but pre-emptive vaccination adds substantial additional benefits on top of self-isolation.

Should the virus be confirmed to be capable of human-to-human transmission, Moghadas says they want to limit the chain of transmission and minimize the risk of evolution of the virus to become more adapted to human conditions. For now, he says, when cases are identified, the person should self isolate immediately. For the authorized vaccine, it should be meted out quickly to target populations, but that could take several weeks to have population level effectiveness.

“Timely action is a critical part of controlling the spread. Self-isolation of symptomatic cases has a significant effect, but that comes with the caveat that we don't know if everybody who is infected will develop symptoms,” says Moghadas.

“There could be potential asymptomatic cases we don't identify and by the time we do identify them, they've been already infecting others in the chain of transmission. This case in B.C. was particularly concerning because they could not find the source of infection.”

(Continue . . . )

There are obviously some formidable barriers to a pre-emptive vaccination campaign of farmers (poultry, and in some places, dairy) and their household contacts.   

• Existing vaccine supplies are quite limited, and an argument could be made that vaccine supplies should be reserved for `essential workers' (e.g. lab workers, HCWs, military, etc.) after an outbreak occurs. 

• Getting farmers to take the seasonal flu vaccine to prevent `reverse zoonosis' or worse - a co-infection with seasonal and zoonotic influenza - has been a challenge (see Canada: BC Provincial Health Officer Urges People Living/Working On Poultry Farms To Get Seasonal Flu Vaccine).

• Only 1 in 3 Canadians age 18-64 got the flu shot last year. It seems likely that an H5N1 vaccine would be an even harder `sell', particularly before any  outbreak has happened. 

• Farmers and governments may shy away from any program that hints that the food supply may be contaminated or unsafe, and `paralysis by analysis' may further delay any decisions. 

Despite its advantages, it seems likely that something would have to substantially elevate the public's - and the government's - level of concern before a pre-emptive vaccine program could be successfully launched. 

California: SLO County Reports Dead Sea Lion From H5N1

 
California Marine Mammals with H5N1

#19,099

Just over a month ago the Año Nuevo Natural Reserve in San Mateo County, CA announced the first U.S. detection of H5N1 in Elephant Seals after dozens of seals were observed either sick or dying and the virus was confirmed by the USDA’s NVSL lab.

Since then, according to local news outlets (see CIDRAP report), the number of HPAI positive seals has increased, along with an H5 positive otter and a sea lion. The actual count is likely much higher, as only a limited number of mammals have been tested.

In early March we saw an update and health advice from California's DPH and just over a week ago a paper on the impact of HPAI H5 on pinnipeds (see UC Davis: High pathogenicity Avian Influenza in Pinniped conservation).

Late yesterday, SLO (San Luis Obispo) county announced the discovery of a dead sea lion (H5N1 positive) on Morro Strand State Beach - some 150 miles south of the first outbreak - in the following press release.

Deceased Sea Lion Found in San Luis Obispo County Tests Positive for H5N1 (Bird Flu)

Author: Public Health
Date: 3/27/2026 2:07 PM

While the risk of H5N1 to the public remains low, officials ask community members and visitors to avoid approaching marine mammals and seabirds and report them instead through the appropriate channels. En español

San Luis Obispo, Calif— A deceased sea lion found along Morro Strand State Beach has tested positive for H5N1 (avian influenza or bird flu), marking the first confirmed case in a marine mammal in the county.

While the risk of H5N1 to the public remains low, officials from San Luis Obispo County Public Health Department, the California Department of Fish & Wildlife, and the Central California Marine Animal Response Team urge community members and visitors to avoid approaching marine mammals and seabirds. This is especially important for animals that appear sick, injured, or deceased.

“Staying 150 yards away from all marine mammals and seabirds; keeping children and pets away from sick, injured or dead wildlife; and not approaching, touching or attempting to assist marine mammals or seabirds,” states guidance from the California Department of Public Health.

Officials ask the public not to touch or help wildlife and instead to call: 

• NOAA West Coast Marine Mammal Stranding Hotline at (866) 767-6114  for assistance with marine mammals
• California Department of Fish and Wildlife at 916-358-2790 for assistance with birds

“While the risk to the public remains low, it’s understandable that people may have questions about their own health if they find themselves unexpectedly exposed to a sick or dead animal,” said County Health Officer, Dr. Penny Borenstein. “If community members have questions about H5N1 as it relates to human health, our Public Health Department can help answer those questions.”

Community members can reach the County of SLO Public Health Department by calling 805-781-5500. To learn more about H5N1 in California by visiting the California Department of Public Health’s bird flu webpage or by reading updates from UC Davis regarding recent H5N1 activity in marine mammals.

History of bird flu in SLO County

Positive cases of bird flu have been found in SLO County since 2022 from various wild birds within the county. In 2024, many states, including California, experienced outbreaks of H5N1 in dairy cattle. There were 38 confirmed human cases in California linked to exposures from infected cattle between September 2024 and January 2025 with none in SLO County.

UC Davis maintains an H5 Influenza A Outbreak in Marine Mammals webpage which now lists 28 Elephant Seals, 2 Sea Lions, and 1 otter as having tested positive in California over the past 6 weeks.

Compared to what we've seen in South America, this remains a small outbreak, but it is likely we are only seeing the tip of the iceberg. Many animals may have died unnoticed at sea, or have washed up on remote beaches. 

Although H5N1 has yet to crack the `transmission code' in humans, evidence suggests marine mammals may be closer to achieving sustained transmission (see Preprint: Massive outbreak of Influenza A H5N1 in elephant seals at Peninsula Valdes, Argentina: increased evidence for mammal-to-mammal transmission).

While that would be a terrible thing for marine mammals, it might not work out too well for us, either. 

Viral Creep: H5N5 Update

 

Cell Reports: Multiple Transatlantic Incursions of HPAI 

clade 2.3.4.4b A(H5N5) Virus into North America and

 Spillover to Mammals (2024)

#19,098

While H5N1 remains the overwhelmingly dominant HPAI H5 subtype reported around the globe, it wasn't so very long ago (2014-2020) that HPAI H5N8 was the preeminent strain, and we continue to see other subtypes bubbling up around the globe. 

• Last month, in South Korea: H5N9 Rising, we looked at concerns over the arrival this winter of a triad of HPAI H5 viruses in Korea; H5N1, H5N6, and H5N9.

• In Mexico, HPAI H5N2 has emerged, infecting both humans and poultry (Preprint: Emergence of a Novel Reassorted HPAI A(H5N2) Virus Associated with Severe Pneumonia in a Young Adult).

• And in both Europe and in Eastern Canada, we've seen sporadic reports of HPAI H5N5 (see Cell Reports: Multiple Transatlantic Incursions of HPAI clade 2.3.4.4b A(H5N5) Virus into North America and Spillover to Mammals) since 2023.

Prior to 2025, H5N5 had only rarely been reported in the United States; mostly in seabirds along the Atlantic coast. Of 18 reports to the USDA, 16 had come from Massachusetts, 1 from Maine, and 1 from New York.  

Based on USDA Data - Graph created with Gemini

While the number of H5N5 detections in the United States remains small, we've witnessed several notable changes in the behavior and ecology of the virus over the past 12 months, not the least of which was the first (fatal) human infection reported last November in Washington state. 

While details remain scant, the local authorities reported:

The person had a backyard flock of mixed domestic birds. DOH testing identified avian influenza virus in the environment of the flock, making exposure to the domestic poultry, their environment, or wild birds the most likely source of exposure for this patient

This human infection was all the more surprising since the virus has never been reported this far west in wild or migratory birds.  Even in Canada, the virus has only been reported (once) as far west as Saskatchewan.

Most of our information comes from the USDA's Detections of HPAI in Wild Birds, which added 26 new detections in 2025, and 2 so far in 2026. 

Based on USDA Data - Graph created with Gemini

The most notable change - starting last summer -  is abrupt shift of detections from the Atlantic seaboard to the Central and Mississippi Flyways (Wisconsin, Iowa, Missouri), with outliers in Florida, Pennsylvania, Mississippi, and Virgina.

While the virus had been primarily reported in coastal and seabirds in 2023-2024, we have also seen a decided shift towards migratory waterfowl (Canadian & Snow Geese) in the latter half of 2025 and in early 2026. 

Based on USDA Data - Graph created with Gemini

At the same time we've seen a shift in the USDA's strain characterization - going from exclusively EA H5N5 prior to last fall, to EA/AM H5N5 since November. 

Based on USDA Data - Graph created with Gemini

The USDA defines these two strains as:

EA = Eurasian; AM = North American; the EA H5 (2.3.4.4) viruses are highly pathogenic to poultry.

EA/AM: reassortant of H5 goose/Guangdong and North American wild bird lineage

It is admittedly difficult to come to any conclusions about the trajectory of H5N5 based on the limited data we have. By all accounts, it remains a minor - albeit persistent - player in the avian flu world. 

But it has demonstrated its zoonotic potential, infecting both humans and other mammalian species, and it continues evolve - reassorting with North American LPAI viruses - as it expands both its geographic and avian host ranges. 

All of which makes HPAI H5N5 well worth our continued attention. 

PLoS Med.: Association Between COVID-19 Vaccination and Sudden Death in Apparently Healthy Younger Individuals:

 
Asystole

#19,097

(Spoiler alert: They didn't find any)

Long before the first COVID vaccine was rolled out to the public (Dec 2020) we were seeing evidence that SARS-CoV-2 infection was driving a sharp spike in sudden cardiac deaths. 

In early April 2020, the NYC Fire Department reported a 400% increase in sudden cardiac arrest death calls beginning in late March (see NBC affiliate Massive Spike in NYC ‘Cardiac Arrest’ Deaths Seen as Sign of COVID-19 Under counting).

Two months later, JAMA published an original investigation which found 10-fold increase in out-of-hospital cardiac arrests in New York City during the peak of their COVID-19 epidemic.

Since then, the evidence of serious cardiac damage from COVID infection has only increased (see European Society of Cardiology: Major Consensus Statement Released on Long-Term Cardiovascular Impact of COVID Infection).

Yet somehow, the COVID vaccine has been made the villain by countless internet influencers, conspiracy theorists, and clickbait-driven social media accounts.  As a result, today COVID infection is treated as trivial by most of the public, while the vaccine is largely shunned. 

While no vaccine can claim to be 100% safe in 100% of the population, serious side effects from the COVID vaccine remain rare, and vaccination has been shown to reduce both deaths and disability from COVID (see ECDC Rapid Review: Does COVID-19 Vaccination Reduce the Risk and Duration of Post COVID-19 Condition?).

Meanwhile, COVID still kills tens of thousands of Americans each each year (see EID Journal: Thrombotic Events and Stroke in the Year After COVID-19 or Other Acute Respiratory Infection). But we no longer count or report COVID deaths, so few take notice. 

Instead, social media seems to focus on every young sudden cardiac death, and tries to link it to receipt of the COVID vaccine.  

While I doubt any amount of scientific evidence is going to sway public opinion, we've another study which finds no credible link between receipt of the COVID vaccine and sudden cardiac death. 

They did, however, find a strong link between recent COVID infection and an increased risk of sudden cardiac death. They authors reported:

 A documented positive SARS-CoV-2 PCR test within 90 days of the index date was associated with higher odds of death (aOR = 2.36; 95%CI [1.84,3.02]; p<0.001), while a positive SARS-CoV-2 PCR test greater than 90 days before the index date was associated with lower odds of death (aOR = 0.83; 95%CI [0.72,0.95]; p=0.006).  

 I've reproduced the author's summary below. Follow the link to read it in its entirety.  I'll have a bit more after the break. 

Association between COVID-19 vaccination and sudden death in apparently healthy younger individuals: A population-based case-control study

Husam Abdel-Qadir  ,Hardil Anup Bhatt ,Sarah Swayze,Michael Paterson,Dennis T. Ko,David N. Juurlink,Jeffrey C. Kwong

Published: March 19, 2026

https://doi.org/10.1371/journal.pmed.1004924

Background

COVID-19 vaccines can cause rare but serious adverse events such as myocarditis and immune thrombotic thrombocytopenia. Despite a lack of strong evidence, concerns have been expressed that COVID-19 vaccination might lead to sudden death in younger healthy adults. We studied the association between COVID-19 vaccination and sudden death in apparently healthy people aged 12–50 years.

Methods and findings

We conducted a population-based case-control study using linked administrative datasets of residents of Ontario, Canada who were alive as of April 1, 2021. We excluded individuals aged >50 years and those with documented cardiovascular disease, mental illness, or diseases that predispose to adverse outcomes from COVID-19. We defined cases as those with out-of-hospital death, or death within 24 hours of presentation to hospital with a final diagnosis of cardiac arrest between April 1, 2021 and June 30, 2023. We matched each case with five controls on age, sex, region of residence, and neighborhood income quintile. We used conditional logistic regression to assess the association between sudden death and previous COVID-19 vaccination after adjusting for multiple potential confounders (positive severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2] tests, number of SARS-CoV-2 polymerase chain reaction (PCR) tests, influenza vaccination, common comorbidities). Sensitivity analyses were conducted with different definitions of the exposure and subsets of cases (with their matched controls). Another sensitivity analysis utilized a modified self-controlled case series (SCCS) of vaccinated individuals meeting the case definition during the study period with up to three doses of any COVID-19 vaccine.

Of 6,365,451 eligible individuals, we identified 4,963 (0.08%) cases meeting our definition of sudden death (median age 36 years, 74.4% male). In the primary analysis, COVID-19 vaccination was associated with a lower risk of sudden death (adjusted odds ratio [aOR] = 0.57; 95% confidence interval (CI) [0.53,0.61]; p < 0.001). The findings were consistent for COVID-19 vaccination within six weeks before death (aOR = 0.63; 95%CI [0.55,0.72]; p < 0.001) and in sensitivity analyses limited to people aged <40 years (aOR = 0.53; 95%CI [0.48,0.58]; p < 0.001), those who died in hospital or in the emergency department (aOR = 0.71; 95%CI [0.55,0.91]; p = 0.006), and after exclusion of opioid-related deaths (aOR = 0.57; 95%CI [0.51,0.64]; p < 0.001).

The SCCS sensitivity analysis showed no significant difference in the rate of sudden death in the 6 weeks following first (relative incidence (RI) 0.87; 95%CI [0.54,1.40]; p = 0.57), second (RI 0.94; 95%CI [0.57,1.57]; p = 0.82), or third (RI 0.87; 95%CI [0.37,2.05]; p = 0.10) dose of the COVID-19 vaccine. Study limitations include the inability to confirm the cause of out-of-hospital deaths and residual confounding due to differences in health-seeking behaviors for the case-control analysis.

Conclusions

These findings do not support the hypothesis that COVID-19 vaccines increase the risk of sudden cardiac death in young healthy adults.

Author summary

Why was this study done?

COVID-19 vaccines were received by a large segment of the population as part of the public health response to the pandemic

There are emerging concerns that COVID-19 vaccines are responsible for sudden death in younger healthy individuals despite a lack of evidence to support this claim

What did the researchers do and find?

A case-control study was conducted involving Ontario residents aged 12–50 years without documented comorbidities predisposing to premature death between April 1, 2021 and June 30, 2023 to examine the association between COVID-19 vaccination and sudden death

The primary outcome was sudden death; the exposure of interest was any COVID-19 vaccination

Among 6,365,451 eligible individuals, 4,806 cases who experienced sudden death were matched to 24,030 controls who were alive on the date of sudden death for each corresponding case

Receipt of COVID-19 vaccination was not associated with increased odds of sudden death

What do these findings mean?

These findings do not support the hypothesis that COVID-19 vaccines increase the risk of sudden cardiac death in younger healthy adults

A limitation of this study was the inability to confirm the cause of out-of-hospital deaths

       (Continue . . . )

Three months ago, in WHO Statement: COVID-19 Still Causes Severe Disease & Renewed Vaccination Recommendations, we looked at the ongoing legacy of COVID infection, and yet another plea for the public to protect themselves. 

In 2025, we also looked at a number of studies which continue to show long-term impacts from (even mild) COVID infection. A few of many include:

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

Preprint: Incidence of Long COVID Following Reinfection with COVID-19

 

BMC Neurology: Long-term Neurological and Cognitive Impact of COVID-19: A Systematic Review and Meta-analysis in over 4 Million Patients

Brain, Behavior & Immunity: COVID-19 may Enduringly Impact Cognitive Performance and Brain Haemodynamics in Undergraduate Students

But apparently it's going to take more than just an abundance of rigorous scientific research to sway public opinion.  

Italy: MOH Statement on First LPAI H9N2 Human Case in Europe (imported)

 
#19,096

With a hat tip to Pathfinder on FluTrackers who posted a media story, I've tracked down the Italian MOH's statement on what appears to be the first H9N2 human infection to be reported in Europe.

Details are vague, but this appears to have been someone who recently arrived from a `non-European' country where they were likely exposed. 

At least one media report identifies the patient as a 'a boy who returned from Africa and was hospitalised a few days after his arrival at Milan Malpensa', although I have yet to find official confirmation.

First the MOH's statement, after which I'll have a bit more. 

(Translated)

Influenza A (H9N2) virus case identified in Lombardy. Routine surveillance and prevention procedures activated.

Press release number 8
Press release date March 25, 2026

The Ministry of Health informs that the Lombardy Region has identified a case of infection with the low-pathogenicity avian influenza A(H9N2) virus of animal origin, in a frail person with concomitant illnesses, who came from a non-European country where he contracted the infection, and is currently hospitalized.

This is the first human case of H9N2 avian influenza detected in Europe. Based on the scientific information available to date, infection occurs through direct exposure to infected poultry or contaminated environments or materials. Human cases are characterized by mild illness, and human-to-human transmission has never been reported.

All the required checks were promptly carried out and the relevant contacts were identified, as part of the ordinary prevention and surveillance activities.

The Ministry of Health immediately activated coordination with the Lombardy Region, the Istituto Superiore di Sanità, and the national reference laboratory expert group, and ensured the coordination and updating of the relevant international bodies.

Currently, no critical issues have been identified and the situation is being constantly monitored.

While 90% of the human H9N2 cases reported have come from China, we've seen sporadic cases in places like Cambodia, Vietnam, and India. We've also seen a handful of cases in Africa (4 in Egypt, 1 in Senegal, and the most recent in Ghana).

The Asian Y280/G57 lineages have shown increasing signs of mammalian adaptation (see EM&I: Enhanced Replication of a Contemporary Avian Influenza A H9N2 Virus in Human Respiratory Organoids)), while the African and Middle Eastern Lineages (mostly European G1-like) are older and less evolved.

Worth noting, LPAI H9N2 has also been detected in African bats (see Preprint: The Bat-borne Influenza A Virus H9N2 Exhibits a Set of Unexpected Pre-pandemic Features).

Given their relatively scarcity outside of Asia, WGS (Whole Gene Sequencing) and antigenic characterization of this latest case will be of considerable interest.

Nature Comms: Post-pandemic Changes in Population Immunity Have Reduced the Likelihood of Emergence of Zoonotic Coronaviruses

Photo Credit NIAID

#19,095

One of the key questions in our post-pandemic world is how much immunity have we gained against other emerging coronaviruses due to our continual exposure to  SARS-CoV-2 and/or COVID vaccines over the past 6 years?

It is not an easy question to answer for a lot of reasons, including:

• First and foremost, COVID is a sarbecovirus, which is just one type of coronavirus. MERS-CoV is another (Merbecovirus), which uses an entirely different receptor cell, making cross-immunity unlikely. 

• Second, even among sarbecoviruses there are at least 4 lineages (clade 1a, 1b, 2, & 3); SARS-CoV was clade 1a, while SARS-CoV-2 was clade 1b. Clades 2 & 3 are more antigenically distant.

• Third, COVID immunity wanes relatively quickly.  While re-exposures or vaccinations can `boost' immunity, variant drift and/or declining vaccine uptake could erode immunity. 

In other words, there are a lot of moving parts.  But in a world teeming with literally dozens of known coronaviruses circulating in the wild (see here, here, here, and here), any cross-immunity is better than none. 

All of which brings us to a new study, published in Nature Comms, where researchers tested cross‑neutralisation of four zoonotic sarbecovirus spikes using human sera from naïve, infected, vaccinated, and hybrid immunity groups.

All four test viruses (SARS‑CoV, Rs4084, GX/P1E, RaTG13) belonged to either clade 1a or 1b, had relatively high spike similarity to SARS-CoV-2, and known ACE2 usage.  Of the 4 tested, the 2002 SARS-CoV showed the least cross-neutralization (30%), while RaTG13 showed the most (79%).

Whether, and how much, cross immunity would extend to more antigenically distant - or non-ACE binding - coronaviruses is unknown. 

What they found was cross‑neutralisation increases with number of vaccine doses and is highest in hybrid immunity (hx of infection & vaccination).  They write:

The highest levels of cross-neutralisation were consistently observed in patients with hybrid immunity, suggesting that vaccine breakdown infections by immune evasive SARS-CoV-2 variants may have a strong protective effect against SARS-CoV-X infection, and that vaccination should be encouraged even in patients with a history of prior infection. In unvaccinated individuals with a history of infection, the strength of cross-neutralisation was lower than in patients with hybrid immunity and was determined by the SARS-CoV-2 infecting strain.

This is consistent with findings that protection conferred by natural infection varies over time and is influenced by the antigenic evolution of SARS-CoV-2, with pre-Omicron infections offering durable immunity, and immunity following Omicron infection waning more rapidly, likely due to increased immune escape38. 

Using this data, the researchers then modeled how COVID and a hypothetical new “SARS‑CoV‑X” might spread in a population like Scotland’s, and determined that our current global immunity makes it more difficult for a SARS-like (ACE-receptor-using) virus to establish itself compared to a naïve community.

Not impossible, but apparently it provides a non-trivial barrier. 

This is a detailed, complex, and nuanced study and deserves to be read in its entirety, as I've only scratched the surface. I'll have a bit more after the break.

Post-pandemic changes in population immunity have reduced the likelihood of emergence of zoonotic coronaviruses

Ryan M. Imrie, Laura A. Bissett, Savitha Raveendran, Maria Manali, Julien A. R. Amat, Laura Mojsiejczuk, Nicola Logan, Andrew Park, Marc Baguelin, Mafalda Viana, Brian J. Willett & Pablo R. Murcia 

Nature Communications volume 17, Article number: 2248 (2026) Cite this article

Abstract

Infections by endemic viruses, and the vaccines used to control them, often provide cross-protection against related viruses, potentially altering the transmission dynamics and likelihood of emergence of new zoonotic viruses with pandemic potential. Here, we investigate how population immunity after the COVID-19 pandemic has impacted the likelihood of emergence of a novel sarbecovirus, termed SARS-CoV-X. 

To this end, we combined empirical cross-neutralisation data with mathematical modelling to identify key immunological and epidemiological factors shaping sarbecovirus emergence. We show that sera from individuals with different COVID-19 immunological histories contained cross-neutralising antibodies against the spike (S) protein of multiple zoonotic sarbecoviruses. 

Simulations parameterised by these data predict that the likelihood of emergence of a novel sarbecovirus has been reduced significantly by population cross-immunity, with outcomes determined by the extent of cross-protection and R0 of the novel virus.

Preventative vaccination against SARS-CoV-X using available COVID-19 vaccines can help resist emergence even in the presence of co-circulating SARS-CoV-2. However, a theoretical vaccine with high specificity to SARS-CoV-2 can increase emergence probability by suppressing SARS-CoV-2 prevalence and, by extension, levels of natural cross-protection. 

Overall, SARS-CoV-2 circulation and vaccination have generated widespread immunity against related sarbecoviruses, creating an immunological barrier to novel sarbecovirus emergence in humans.

(Continue . . . ) 

While these findings appear to do little to negate the threat from MERS-CoV (see (Referral) Nature: Human MERS-CoV cases are falling but pose an ongoing pandemic threat), it does suggest that we likely now carry some degree of protection against a narrow - but important - range of sarbecoviruses.

While not exactly a get-out-of-pandemic-free card - when it comes to emerging viruses - any amount of immunity has to beat no immunity at all.