While influenza often takes a back seat during the summer, this year - with H5N1 spreading globally, and recent reports of spillovers of swine variant viruses in Brazil and Taiwan - influenza remains very much a hot topic.
Add in that it is now winter in the Southern Hemisphere, and some regions are reporting moderate-to-high activity, and there are good reasons for the following updates from the CDC.
First stop, the CDC's COCA Call webinar (and an accompanying 78 slide PDF file) on What Providers Need to Know about Zoonotic Influenza, both released on June 20th, is now online and available to view and/or download.
A week ago, in WHO Risk Assessment: Fatal H1N1v Infection In Brazil, we saw the report of a fatal swine H1N1 infection in an immunocompromised woman who lived near - but had no direct contact with - a pig farm in Brazil.
While severe and/or fatal infections with swine variant viruses are rare, we have seen them reported previously (see J. Virology: Analysis Of A Swine Variant H1N1 Virus Associated With A Fatal Outcome). And while most people contract these viruses from direct contact with pigs, limited human-to-human spread has been reported.
She did have contact with two reportedly asymptomatic workers from that farm, however.
June 23, 2023 – On June 7, 2023, the Brazilian Ministry of Health (MoH) reported to the World Health Organization (WHO) a fatal human infection with a swine influenza A(H1N1)v virus, a virus that typically spreads in pigs and not people. A clinical specimen collected from the patient is being sent to the United States Centers for Disease Control and Prevention (CDC) for confirmation and further characterization of the virus. While not common, human infections with swine influenza viruses do happen. The specimen is being sent to CDC as the agency’s Influenza Division is one of seven World Health Organization Influenza Collaborating Centers that support global influenza surveillance.
Testing conducted by the Brazil National Influenza Center indicated the virus is genetically related to swine influenza viruses detected previously in swine in Brazil and to previous human infections with A(H1)v viruses detected in Brazil in 2020, 2021, and 2022. CDC will perform additional sequencing of the virus genome in order to look for potential mutations that might make this virus spread more easily from person-to-person or cause more severe disease in people. If virus isolation is successful, additional virologic characterization may be performed.
The patient, who reportedly was severely immunocompromised, lived near a swine farm and had close contact with two family members who worked at the farm. The patient developed symptoms on May 1, was hospitalized with symptoms of an acute respiratory infection on May 3, and died on May 5. People with weakened immune systems are more likely to become seriously ill from any influenza virus infection, including swine influenza viruses, which most often result in mild illness. Investigations conducted by the Brazilian MoH reported that both family members who worked at the swine farm did not develop respiratory symptoms and also tested negative for influenza. In addition, no person-to-person spread has been identified in this case.
While this is the first influenza A(H1N1v) infection reported in Brazil in 2023, sporadic human infections have occurred in the past.
Sporadic infections and even localized outbreaks among people with variant influenza viruses can occur. When this happens, these are called variant virus infections and denoted with the letter “v” in the virus name. In the U.S., 18 influenza A(H1N1)v variant virus infections have been reported since 2010. There have been 475 human infections with other variant flu viruses during that same time.
Most commonly, human infections with variant viruses occur in people with exposure to infected pigs (e.g., children exposed to pigs at an agricultural fair, people who raise pigs, or workers in the swine industry). CDC has issued guidance for people attending settings where swine might be present, including additional precautions for people who are at higher risk of serious flu complications.
This isolated sporadic variant virus infection in Brazil is not thought to pose a risk to the U.S. public. In general, the current risk to the U.S. general public from swine influenza is low, but all influenza viruses have the capacity to change, and that is why it is important to follow up on each of these variant virus infections. CDC continues to monitor closely for variant influenza virus infections and will report cases weekly in FluView and in the Novel Influenza A Virus Infections (cdc.gov) section of FluView Interactive.
No clear patterns have emerged at this time
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As flu activity in the United States continues at low levels that are typical for this time of year, CDC also is tracking flu activity in the Southern Hemisphere, which generally has its “flu season” at this time. Surveillance data show that several Southern Hemispheric countries are currently experiencing higher or earlier flu activity compared to what was seen prior to the COVID-19 pandemic; however, activity varies by country or region.
According to flu activity data submitted to the World Health Organization:
Limited data are available on flu-related hospitalizations in Southern Hemisphere countries at this time, but South Africa has reported a “moderate” level of flu related hospitalizations, while Australia has reported a “normal” level of flu-related hospitalizations compared with historic trends.
- Influenza A(H1N1) viruses have been most commonly reported.
- Chile has reported an early start to their flu season.
- Argentina is experiencing typical levels of flu activity.
- South Africa has been experiencing high flu activity for this time of year.
- Australia is experiencing typical levels of flu activity.
- While not in the Southern Hemisphere, Mexico has been experiencing abnormally high flu activity in May and June. Typically, Mexico has low flu activity during this time of the year.
Flu season in the Southern Hemisphere usually occurs between April and September, compared with October through May in the Northern Hemisphere. Experts often look at Southern Hemispheric flu activity for clues as to what could happen in the United States. This 2023 Southern Hemisphere flu season, some Central and South America countries have early or intense influenza A(H1N1) activity, while others have typically timed seasons with a mix of influenza A(H3N2), influenza A(H1N1), and influenza B/Victoria.
What happens in the Southern Hemisphere does not necessarily predict what will happen next in the Northern Hemisphere because different influenza viruses may predominate in different parts of the world and immunity may be different between populations. Nevertheless, flu seasons in the Southern Hemisphere remind us of what could happen in the United States and help us think of scenarios to better prepare for our flu season.
While flu activity is always challenging to predict, it has been especially unpredictable since the COVID-19 pandemic. The continued impact of COVID-19 on the circulation of respiratory viruses like influenza, remains unclear, but ongoing global flu surveillance remains important for trying to understand flu virus seasonality going forward. CDC will continue to monitor ongoing flu activity in the Southern Hemisphere as their season progresses.
With COVID still churning out new variants, the increased threat from zoonotic flu cases, and the possibility that something `new' might emerge, it is important to monitor influenza-like illness around the globe closely.
Unfortunately, surveillance and reporting remains suboptimal in many regions (see Lancet Preprint: National Surveillance for Novel Diseases - A Systematic Analysis of 195 Countries), and international tensions and economic priorities often conspire to further cloud the picture (see Flying Blind In The Viral Storm).
Which is why we need to be preparing for the next pandemic as if the warning bells were already ringing.