Flu Vaccine Effectiveness Good This Season, CDC Says

This season's influenza vaccine effectiveness is 45% overall and 55% in children, according to preliminary estimates from the Centers for Disease Control and Prevention (CDC). Vaccine effectiveness (VE) was higher against influenza B/Victoria (50%) and considerably lower against influenza A(H1N1)pdm09 (37%).

"Vaccination is providing substantial protection (VE = 55%) for children, who have been particularly hard hit by flu this season," the authors write. This season, influenza-like illness activity started early, during the week ending November 9, 2019, and 92 children have died so far.

Fatimah S. Dawood, MD, Influenza Division, National Center for Immunization and Respiratory Diseases, CDC, and colleagues present preliminary estimates of influenza vaccine effectiveness in an article published online today in Morbidity and Mortality Weekly Report.

"This paper supports the fact that the vaccine is very effective for children under 18 years of age, and vaccinated children are far much less likely to get influenza and far much less likely to die from influenza," Infectious Diseases Society of America spokesperson Aaron Glatt, MD, told Medscape Medical News. It's very important for parents and others to know this information, said Glatt, who is also a professor and chair of medicine at Mount Sinai South Nassau, Oceanside, New York.

The vaccine was most effective among children aged 6 months to 17 years and least effective among adults aged 18 to 49 years. The lowest effectiveness — 5% — was seen for influenza A(H1N1)pdm09 among adults aged 18 to 49 years.

The seasonal influenza "vaccine has significantly reduced medical visits associated with influenza so far this season," the authors add.

Dawood and colleagues analyzed data from the US Influenza Vaccine Effectiveness Network, which included 4112 children and adults with acute respiratory illness between October 23, 2019, and January 25, 2020. Using data from five sites (Michigan, Pennsylvania, Texas, Washington, and Wisconsin), they estimated the vaccine's effectiveness "for preventing medically attended, laboratory-confirmed influenza virus."

Overall effectiveness against influenza A and B was 45%, after adjustment for study site, age group, sex, race/ethnicity, self-rated health status, days from illness onset to enrollment, and month of illness. Vaccine effectiveness was 55% among children and adolescents aged 6 months to 17 years; 25% among adults aged 18 to 49 years; and 43% for those aged 50 years and older.>

Overall vaccine effectiveness was 50% against influenza B/Victoria — 56% among children aged 6 months to 17 years and 32% among adults aged 18 to 49 years.

Overall vaccine effectiveness was 37% against A(H1N1)pdm09. Effectiveness was 51% among children and adolescents aged 6 months to 17 years, 5% among adults aged 18 to 49 years, and 50% among those aged 50 years and older.

"Interim VE estimates are consistent with those from previous seasons, ranging from 40%-60% when influenza vaccines were antigenically matched to circulating viruses," the authors explain.

Regarding the 37% effectiveness against A(H1N1)pdm09, Glatt said more than 14,000 people have died from influenza in the United States this season. "If you had a magic pill . . . that would prevent a third of those, who wouldn't take that?" he asked. "Thirty-seven percent is still a wonderful number; this is an effective, good vaccine that's very safe." He added that it is not only preventing illness, but also possibly lessening the severity of illness, reducing mortality, and protecting others from becoming ill.

Limitations of Findings

Limitations of the report's findings include sample sizes that were too small to estimate overall effectiveness against illnesses linked to A(H3N2) virus infections. In addition, vaccine effectiveness estimates could change at the end of the season as more data become available or if circulating viruses change later this flu season.

Also, vaccination status was self-reported at four of the five study sites, which may have resulted in some patients having their vaccination status misclassified, and observational study designs have the potential for bias and confounding.

The authors also note that the vaccine effectiveness estimates only address the prevention of outpatient medical visits and not more severe complications, including hospitalization and death. Data from studies of vaccine effectiveness for the prevention of more severe outcomes will be available later.

Clinicians should remind patients that it is not too late to get vaccinated, the vaccine is safe, and it can prevent death, Glatt said.

Regarding antiviral medications, "CDC recommends antiviral treatment for any patient with suspected or confirmed influenza who is hospitalized, has severe or progressive illness, or is at high risk for complications from influenza, including children aged < 2 years and adults aged ≥ 65 years, regardless of vaccination status or results of point-of-care influenza diagnostic testing," the authors explain.

The authors and Glatt have disclosed no relevant financial relationships.

MMWR. Published online February 20, 2020. Full text

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