How Effective is the 2019-20 Flu Shot?

February 25, 2020
Grant M. Gallagher

Based on data from 4112 children and adults, vaccine effectiveness was estimated to be 37% against influenza A and 50% against influenza B viruses.

The United States appears to be experiencing the peak of influenza season and one which featured the earliest start since the 2009 H1N1 pandemic. While vaccination is recommended every year for all individuals aged 6 months or older, the effectiveness of the seasonal influenza vaccine varies annually.

The US Centers for Disease Control and Prevention’s (CDC) latest Morbidity and Mortality Weekly Report features interim estimates of effectiveness for the 2019-20 seasonal influenza vaccine. According to the report, the overall estimated effectiveness of seasonal influenza vaccines is currently 45%.

Estimates were made based on data collected from 5 study sites in Michigan, Pennsylvania, Texas, Wisconsin, and Washington. Patients aged 6 months or older seeking medical care for an acute respiratory infection with cough were enrolled into the research. Participants or their proxies were interviewed for demographic data and to ascertain whether they received the flu shot.

Nasal and oropharyngeal swabs were used to obtain respiratory samples, which were tested with real-time reverse transcription-polymerase chain reaction protocols for identification of influenza viruses.

Participants were considered vaccinated if they had received 1 or more doses of any seasonal influenza vaccine 14 or more days before illness onset.

Based on data from 4112 children and adults, vaccine effectiveness was estimated to be 37% against influenza A and 50% against influenza B viruses. Influenza B was the predominant strain in December, but as of mid-February there was a sharp rise in influenza A cases.

“Interim VE [vaccine effectiveness] estimates are consistent with those from previous seasons, ranging from 40%—60% when influenza vaccines were antigenically matched to circulating viruses,” the authors of the article wrote.

In the study, influenza positive participants had a vaccination rate of 37%, compared with a 55% vaccination rate among influenza negative participants. This points to an association between vaccination and reduction of influenza risk.

“I think it gets lost in the narrative that while the flu shot prevents a percentage of infections, and some people will go and develop the flu anyway, even if they do, there's some evidence that it's attenuated, and that the complications from having the flu including death are lessened even if someone develops flu after they've gotten a shot,” Jason Gallagher, PharmD, FCCP, FIDSA, BCPS, clinical professor at Temple University College of Pharmacy and editor-in-chief of Contagion® said in an interview.

The study authors balanced the ongoing importance of annual vaccination with the need for improved vaccines.

“Current influenza vaccines are providing substantial public health benefits; however, more effective influenza vaccines are needed,” the authors wrote.

Still, the authors emphasized, clinicians should continue to vaccinate persons over the age of 6 months who haven’t received their flu shot this season.

“[The] 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.”