Zombie Apocalypse or Just Another Flu Season?

• CDC estimates that the burden of illness during the 2018–2019 season included an estimated 35.5 million people getting sick with influenza, 16.5 million people going to a health care provider for their illness, 490,600 hospitalizations, and 34,200 deaths from influenza (Table 1). 
• The overall burden of influenza for the 2017-2018 season was an estimated 45 million influenza illnesses, 21 million influenza-associated medical visits, 810,000 influenza-related hospitalizations, and 61,000 influenza-associated deaths

Key Updates for Week 10, ending March 7, 2020

Flu activity as reported by clinical laboratories remains high but decreased for the fourth week in a row; however, influenza-like illness activity increased slightly.  Severity indicators remain moderate to low overall, but hospitalization rates differ by age group, with high rates among children and young adults.

Influenza-Associated Hospitalizations 2019-20 Season

The overall cumulative hospitalization rate was 61.6 per 100,000 population which is higher than all recent seasons at this time of year except for the 2017-18 season. Rates in children 0-4 years old and adults 18-49 years old are now the highest CDC has on record for these age groups, surpassing the rate reported during the 2009 H1N1 pandemic. Hospitalization rates for school-aged children are higher than any recent regular season but lower than rates during the pandemic.

• Laboratory confirmed influenza associated hospitalization rates for the overall U.S. population remain moderate compared to recent seasons, but rates for children 0-4 years and adults 18-49 years are now the highest CDC has on record for these age groups, surpassing rates reported during the 2009 H1N1 pandemic.  Hospitalization rates for school-aged children (5-17 years) are higher than any recent regular season but remain lower than rates experienced by this age group during the pandemic.
• Pneumonia and influenza mortality has been low, but 144 influenza-associated deaths in children have been reported so far this season. This number is higher for the same time period than in every season since reporting began in 2004-05, except for the 2009 pandemic.
• CDC estimates that so far this season there have been at least 36 million flu illnesses, 370,000 hospitalizations and 22,000 deaths from flu.

What viruses will the 2019-2020 flu vaccines protect against?

There are many different flu viruses and they are constantly changing. The composition of U.S. flu vaccines is reviewed annually and updated as needed to match circulating flu viruses. Flu vaccines protect against the three or four viruses (depending on the vaccine) that research suggests will be most common. For 2019-2020, trivalent (three-component) vaccines are recommended to contain:

• A/Brisbane/02/2018 (H1N1)pdm09-like virus (updated)
• A/Kansas/14/2017 (H3N2)-like virus (updated)
• B/Colorado/06/2017-like (Victoria lineage) virus

Quadrivalent (four-component) vaccines, which protect against a second lineage of B viruses, are recommended to contain:

• the three recommended viruses above, plus B/Phuket/3073/2013-like (Yamagata lineage) virus.

How well-matched are 2019-2020 vaccine viruses to circulating flu viruses? How well is flu vaccine protecting against illness?

It’s understandable that people want to know how well flu vaccines are working or are expected to work this season. CDC does not have flu vaccine effectiveness estimates for this season yet because it is still early in the season  and these estimates are based on epidemiologic studies comparing illness among vaccinated versus unvaccinated people. That data will be available later.

In the meantime, laboratory data can provide some insight into how well vaccines might work. The most helpful data for this is the antigenic characterization data, which are updated weekly in FluView. Limited antigenic data on recently circulating viruses are available at this point in the season, and the data suggest similarity of the circulating influenza A(H1N1)pdm09 and B/Yamagata viruses tested so far to the vaccine viruses. However, the influenza B/Victoria and A(H3N2) viruses that have been tested show some reduced similarity to the vaccine viruses. Again, at this point in the flu season, the antigenic data are limited and can only give early insights into how well vaccines might work. More information about how CDC antigenically characterizes flu viruses is available, and more complete antigenic data will be available in the coming weeks.

During past seasons when vaccine viruses were antigenically “like” most circulating viruses, vaccine effectiveness in the range of 40% to 60% has been observed. This means that people who get vaccinated may still get sick, but they are about half as likely to get sick as someone who was not vaccinated. Another important thing to remember is that vaccination may make illness less severe in people who get vaccinated and still get sick. In general people who get vaccinated are better off than people who do not get vaccinated. It’s important to remember though, that things can change very quickly with flu and we could still see circulation of flu viruses with significant antigenic drift this season.