Study findings showed that last season’s influenza vaccine “offered no substantial protection” against H3N2 viruses that moved through the United States late in the season, highlighting the ongoing challenge of producing a vaccine that works against this influenza A subtype and leading two experts to wonder if an additional H3N2 strain should be added to the seasonal vaccine.

The emergence of a drifted H3N2 variant in the second half of the season prompted WHO to postpone choosing an H3N2 virus for the Northern Hemisphere’s 2019-2020 influenza vaccines, which ultimately led to a delay in some influenza vaccine shipments in the U.S.