Experts say the 2019-20 influenza season kicked off around the first week of November 2019, when influenza-like illness (ILI) activity first exceeded the national baseline -- signaling the earliest start to the influenza season since the 2009 influenza A(H1N1) pandemic.

During each influenza season, the Centers for Disease Control and Prevention (CDC) estimates seasonal influenza vaccine effectiveness in preventing laboratory-confirmed influenza associated with medically attended acute respiratory illness (ARI). In their interim report, Dawood, et al. (2020) used data from 4,112 children and adults enrolled in the U.S. Influenza Vaccine Effectiveness Network (U.S. Flu VE Network) from Oct. 23, 2019 through Jan. 25, 2020.

Overall, vaccine effectiveness (VE) against any influenza virus associated with medically attended ARI was 45 percent, according to the researchers. VE was estimated to be 50 percent against influenza B/Victoria viruses and 37 percent against influenza A(H1N1)pdm09, indicating that vaccine has significantly reduced medical visits associated with influenza so far this season.

According to Dawood, et al. (2020), "Notably, vaccination provided substantial protection (VE = 55%; 95% CI = 42%–65%) among children and adolescents aged 6 months–17 years. Interim VE estimates are consistent with those from previous seasons, ranging from 40 percent to 60 percent when influenza vaccines were antigenically matched to circulating viruses. CDC recommends that health care providers continue to administer influenza vaccine to persons aged ≥6 months because influenza activity is ongoing, and the vaccine can still prevent illness, hospitalization, and death associated with currently circulating influenza viruses as well as other influenza viruses that might circulate later in the season."

The researchers add, "These interim VE estimates indicating a 45 percent reduction in influenza illness associated with a medical visit so far this season are particularly important in the context of the substantial prevalence of influenza in the United States: during the previous decade, influenza caused an estimated 4.3–21 million doctor visits, 140,000–810,000 hospitalizations, and 12,000–61,000 deaths each year. Among U.S. Flu VE Network participants, influenza virus infections accounted for approximately 25 percent of medically attended visits for ARI, demonstrating the considerable contribution of influenza virus infections to medically attended outpatient visits for ILI this season. Both influenza A and B viruses can cause severe illness, including hospitalizations and deaths. Some studies have suggested that influenza B virus infections might also result in more severe illness among children (4,5). Interim VE estimates indicate that the 2019–20 influenza vaccine protects against the predominant B/Victoria viruses from subclade V1A.3 and are consistent with VE estimates against influenza B/Victoria (range = 49%–56%) during seasons when the B/Victoria component of the vaccine was well matched to circulating viruses."

Reference: Dawood FS, Chung JR, et al. Interim Estimates of 2019–20 Seasonal Influenza Vaccine Effectiveness — United States, February 2020. Morbidity and Mortality Weekly Report. Vol. 69, No. 7. Feb. 21, 2020