Frequently Asked Flu Questions according to the CDC Website:

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.

The World Health Organization (WHO) made the selection of the H1N1 and both B components for 2019-2020 Northern Hemisphere flu vaccines on February 21 and at that time decided to delay the decision on an H3N2 vaccine component. FDA’s Vaccines and Related Biological Products Advisory Committee (VRBPAC) also selected the H1N1 and B components at their first meeting on March 6, but also decided to postpone the selection of the H3N2 component. WHO selected the H3N2 component listed above on March 21, 2019. VRBPAC chose the same H3N2 component for U.S. vaccines on March 22, 2019.

Are there any changes to the 2019-2020 Northern Hemisphere vaccines from what was included in this season’s 2018-2019 U.S. flu vaccines?

Flu vaccines are updated to better match circulating viruses. The A(H1N1)pdm09 vaccine component was updated from an A/Michigan/45/2015 (H1N1)pdm09-like virus to an A/Brisbane/02/2018 (H1N1)pdm09-like virus. The A(H3N2) vaccine component was updated from an A/Singapore/INFIMH-16-0019/2016 A(H3N2)-like virus to an A/Kansas/14/2017 (H3N2)-like virus. Both B/Victoria and B/Yamagata virus components from the 2018-2019 flu vaccine remain the same for the 2019-2020 flu vaccine.

Why was there a delay in selecting the A(H3N2) virus component of 2019-2020 flu vaccines?

A number of factors can make getting a good vaccine virus for vaccine production challenging. H3N2 viruses have presented an increasing challenge for vaccine virus selection due to frequent changes in the H3N2 viruses and difficulties in generating optimal candidate vaccine viruses for use in manufacturing. In February 2019, experts at the vaccine consultation meetings reviewed various sources of data including virus surveillance, antigenic characterization, and virus fitness forecasts, identified multiple co-circulating H3N2 virus groups. These data showed that the proportion of viruses in one antigenically distinct group of H3N2 viruses (called 3C.3a virus) was rapidly increasing in some countries, particularly the United Sates. Selection of an H3N2 vaccine virus for 2019-2020 Northern Hemisphere vaccines was delayed from February to March to allow more time for monitoring H3N2 virus circulation and characterization of potential H3N2 candidate vaccine viruses.

Is it the first time that a WHO recommendation of a component of the seasonal flu vaccine has been postponed?

No. The last time there was a postponed influenza vaccine recommendation was in February 2003, due to challenges selecting the A(H3N2) vaccine component for the 2003-2004 flu season. During the 2002-2003 influenza season, a distinct antigenic group virus emerged and increased in circulation, but it was unclear how fast the increase would be and no appropriate candidate vaccine virus was available at the time. Therefore, the decision was postponed and the recommendation was announced in March 2003.

Will the delay in selecting the H3N2 component delay availability of flu vaccines for the upcoming 2019-2020 northern hemisphere flu season?

It is too soon to say how the delay in the selection of the H3N2 candidate vaccine component may affect the timing of vaccine availability for the 2019-2020 flu season. Private manufacturers in the United States produce flu vaccines each season. Once the viruses are selected for the new vaccine formulation, manufacturers operate under a tight timeline for producing, testing, releasing and distributing flu vaccine. CDC and other federal partners will continue to coordinate and collaborate with U.S. flu vaccine manufacturers to monitor production and vaccine availability timelines