Influenza Vaccine

Vaccine Effectiveness Declined During the End of 2018-2019 Influenza Season

The predominance of A(H3N2) clade 3C.3a viruses during the latter part of the 2018-2019 influenza season was associated with a decrease in vaccine effectiveness, according to results of a new study.

According to the study authors, the findings support the A(H3N2) vaccine component update for the 2019-2020 northern hemisphere influenza vaccines.


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To conduct their study, the researchers first analyzed the distribution of influenza viruses and A(H3N2) clades in the United States before and during the 2018-2019 influenza season using virologic surveillance data from the Centers for Disease Control and Prevention’s US Influenza Vaccine Effectiveness (Flu VE) Network.

The researchers then evaluated vaccine effectiveness against influenza-associated illnesses using data from the Flu VE Network. This network included ambulatory care patients aged 6 months or older with acute respiratory illness who had visited any of 5 sites between November 23, 2018, and May 3, 2019.

The respiratory specimens of 2763 patients were tested for influenza. By comparing the odds of influenza among vaccinated vs unvaccinated patients with a test-negative design, the researchers estimated the vaccine effectiveness.

According to the study authors, A(H3N2) clade 3C.3a viruses caused an increasing proportion of influenza cases during the 2018-2019 influenza season.

Of all the patients, 1325 (48%) were infected with the 2009 pandemic A(H1N1) strain [A(H1N1pdm09)], and 1350 (49%) were infected with A(H3N2). Among the 1054 sequenced A(H3N2) viruses, clade 3C.3a accounted for 977 (93%) of them.

The vaccine effectiveness was 44% against A(H1N1)pdm09, 9% against A(H3N2), and 5% against A(H3N2) clade 3C.3a viruses.

“Annual monitoring of [vaccine effectiveness] with increased use of genetic characterization of circulating influenza viruses supports ongoing efforts to improve influenza vaccines,” the researchers concluded. “…Timely integration of clinical vaccine protection and virologic data may strengthen the evidence base for selecting influenza vaccine viruses and increase the probability that vaccine composition matches the predominant viruses.”

—Colleen Murphy


Flannery B, Garten Kondor RJ, Chung JR, et al. Spread of antigenically drifted influenza A(H3N2) viruses and vaccine effectiveness in the United States during the 2018-2019 season. J Infect Dis. 2020;221(1):8-15.