Influenza Surveillance Update – Alicia Budd (CDC)
Alicia provided an update on influenza activity for Week 1, ending January 4, 2020. Flu activity remains high and widespread across the country. However, indicators of severity are not high.
Clinical labs reported 23% of submitted specimens were positive for influenza. This is a decrease from the 26% positive rate reported from the previous week. It is too early to know if this is a course correction from the holiday elevation resulting from healthcare seeking behavior changes. It also may be due to a decrease in influenza because schools were closed, offering less opportunity for close transmission. This also could be the result of decreased circulation of the B Victoria strain. CDC will continue to monitor these trends closely.
Public health labs reported widespread influenza with a mixture of strains among B Victoria (the most commonly reported), H1, and H3. However, in some parts of the country, B Victoria is beginning to wane. Strain differences continue to be seen among different age groups. For persons 0–24 years of age, B Victoria is predominant, while H1 is reported more frequently in persons 25 and older.
Genetic characterization of influenza viruses continues to increase. Almost 900 specimens have been submitted for testing. H1 and B Victoria specimens continue to be the same clade as the vaccine reference virus. The majority (66%) of characterized H3 viruses belonged to the 3C.2a1 clade/subclade, which varies from the 3B.3a clade used in the vaccine. For B Victoria, 9% of specimens were double deletion viruses, while 91% were triple deletion viruses. Fifty-eight percent (58%) of B viruses were similar to the cell-grown reference virus. No changes in antiviral susceptibility have were seen during the week. Of the more than 800 viruses tested, 99% were susceptible to the antiviral medications oseltamivir, zanamivir, peramivir, and baloxavir. Only 1 H1 specimen was found to be resistant to oseltamivir and zanamivir, and 1 B Victoria specimen was resistant to zanamivir.
Outpatient influenza-like illness (ILI) activity decreased to 5.8%. However, this also may have been influenced by changes to healthcare seeking activity around the holidays. Despite that ILI is still well above national and regional baseline levels. State ILI activity also was high, with 34 states reporting high levels of influenza.
Although lots of influenza is circulating, severity indicators are not unusually high. Cumulative hospitalization rates are similar to other seasons, at 14.6 per 100,000 for the overall population. Highest rates are seen in age 65 and above (33.3 per 100,000), followed by the 0–4 year age group (26.8 per 100,000.) Fifty-five percent (55%) of hospitalizations are due to infection with influenza A. Rates of reported pneumonia and influenza (P&I) are still below threshold levels for the season.
Five (5) influenza-related pediatric deaths were reported during the week, bringing the 2019–2020 total for pediatric deaths to 32. Twenty-one (21) of these deaths were associated with B Victoria, and 11 were influenza A. All of the A infections were subtyped as H1.
Fifty (50) states reported widespread or regional influenza activity. In-season burden of influenza estimates are 9.7 million persons infected, 87,000 hospitalizations, and 4,800 deaths.
Alicia drew attention to 2 publications that were released last week. A health advisory was issued summarizing influenza activity seen this season and emphasizing the importance of early treatment with antivirals. The second publication was an early release MMWR which summarized the early season B Victoria seen in Louisiana. Activity began there in late July with significant increases in B Victoria triple deletion virus being seen. This is what we are now seeing nationwide.
At the conclusion of Alicia’s presentation, one participant asked if we are able to provide a simple answer that can be understood by the public about whether this year’s vaccine is proving effective against circulating strains. Alicia responded that this isn’t really possible at this time, as this is a complicated area. Genetic testing helps us understand the drift of currently circulating viruses compared to those in the vaccine, but vaccine effectiveness (VE) is impacted by more than genetic drift. For example, there can be cross protection or existing protection from prior influenza or influenza vaccine. In spite of recent media reports suggesting this year’s vaccine is not a good match to circulating strains, it is simply too early to tell. Preliminary VE estimates will be published sometime in February. These estimates are based on information collected from sites around the country that are specifically examining this question.
Other Items – L.J Tan
- 2020 Summit Meeting – This year’s meeting will be held on May 18, the day prior to the opening of the National Immunization Conference. Information on registration and other pertinent details is available on the Summit website. Approaching deadlines of importance are: nominations are being accepted for the 2020 Immunization Excellence Awards through February 1, and the deadline for a poster submission is March 15.
- Summit Call Schedule – L.J reminded partners that Summit calls are held each Thursday at 3 pm ET unless otherwise cancelled via an email announcement.