How Well Did the 2018–19 Flu Vaccine Work? CDC's Interim Estimate: 47%

Annual flu vaccination is recommended for everyone 6 months and older, and how well the vaccine works varies by season. Every season since 2004–05, CDC has estimated vaccine effectiveness against lab-confirmed flu that sends people to a doctor with an acute respiratory illness (ARI). This is the interim estimate for 2018–19, from patients enrolled November 23, 2018–February 2, 2019.

How it was measured

• The U.S. Influenza Vaccine Effectiveness Network enrolled outpatients 6 months and older with an ARI with cough of up to 7 days, not yet on antiviral treatment, at five sites: Michigan, Pennsylvania, Texas, Washington and Wisconsin.
• Nose and throat swabs were tested by RT-PCR.
• Vaccinated meant at least one dose of 2018–19 vaccine 14 or more days before illness, from records, registries or self-report, depending on the site.
• Effectiveness compares the odds of vaccination in flu-positive and flu-negative patients (the test-negative design), adjusted for site, age, sex, race and ethnicity, self-rated health, days to enrollment and month.

Who had flu

Of 3,254 patients, 465 (14%) had flu: 456 influenza A and 9 influenza B. Of 394 subtyped A viruses, 293 (74%) were A(H1N1)pdm09 and 101 (26%) A(H3N2). 43% of flu patients had been vaccinated, against 57% of those without flu.

The estimates

In context

• Flu vaccines have usually been about 40%–60% effective in seasons when the vaccine matches circulating viruses. Against H1N1, this was similar to 2015–16 (45%), the last H1N1-dominated season, but lower than a meta-analysis of U.S. seasons since 2010–11, and lower than interim estimates from Canada (72%, 2018–19) and Australia (78%, 2018).
• Possible reasons: small samples in some age groups, regional differences in viruses, and genetic variation within subtypes.
• In children, the H1N1 figure (62%) matched 2015–16 (49%–63%).
• In 2017–18, an H3N2 season, vaccination was estimated to prevent 7.1 million illnesses, 3.7 million medical visits, 109,000 hospitalizations and 8,000 deaths.

What CDC recommends

• Keep vaccinating while flu circulates — anyone 6 months or older not yet vaccinated should be. The vaccine can prevent illness, hospitalization and death from viruses circulating now or later in the season, and can make illness milder.
• Vaccination lowers the risk of flu deaths in children — about 80% of reported pediatric flu deaths in past seasons, including 2017–18, were in unvaccinated children — and of flu hospitalization in pregnancy, and can reduce cardiac events in people with heart disease.
• Antivirals are recommended for anyone with suspected or confirmed flu who is hospitalized, severely ill or at high risk of complications, whatever their vaccination status; they can be considered for healthy outpatients who can start within 48 hours.

Limits

• Smaller samples than usual meant wide confidence intervals, especially for adults 50 and older; end-of-season estimates may change.
• Vaccination was partly self-reported, and the type of vaccine wasn't yet known.
• An observational design is more open to bias than a randomized trial.
• The estimates cover doctor's visits, not hospitalization or death.

Sources

Based on Doyle JD, Chung JR, Kim SS, et al., "Interim Estimates of 2018–19 Seasonal Influenza Vaccine Effectiveness — United States, February 2019," MMWR Morbidity and Mortality Weekly Report, volume 68, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report's discussion gives the effectiveness against A(H1N1)pdm09 as 47%; its results and table give 46%, which is used here.