The idea is simple. Vaccinate as many people as possible when you have the chance as soon as the flu shot becomes available. If you ask people to come back to get their flu shot closer to the flu season (typically November to April in the Northern hemisphere), there is a chance that some of them will not come back, because that's life. In the United States, fewer than half of the population receives the flu shot each year (37% in the 2017-2018 season and 45% in the 2018-2019 season received the flu shot). It remains to be seen whether or not the COVID-19 pandemic would make these numbers worse. Therefore, every effort must be made to improve vaccine uptake. The CDC recommends initiating vaccination campaigns as soon as the vaccine is available (i.e., in July) to allow sufficient time to vaccinate the population and avoid some persons going unvaccinated for influenza.
However, several observational studies have shown that the effectiveness of influenza vaccine wears off within a single influenza season (less than a year). For example, G. Thomas Ray and colleagues evaluated the effectiveness of the inactivated influenza vaccine from September 2010 to March 2017 and found that compared with persons vaccinated 14 to 41 days prior to being tested, persons vaccinated 42 to 69 days prior to being tested had 1.32 (95% CI, 1.11 to 1.55) times the odds of testing positive for any influenza. The odds ratio increased linearly by about 16% for each additional 28 days since vaccination. According to the CDC, peak influenza activity is typically between Christmas and Valentine's day (December to February), more frequently during Valentine's day (15 previous seasons) than Christmas (7 previous seasons). Based on these findings, getting the flu shot in July would essentially increase the odds of testing positive for the flu during Christmas by about 80%, 64% if the flu shot received in August, and 48% if received in September, and 32% if received in October. Of course, observational studies are subject to many limitations and biases. Some studies suggest that the waning occurs to a greater extent with H3N2 influenza A than H1N1 influenza A or influenza B. Other studies suggest that waning is greater among older adults and younger children.
On average, it takes 2 weeks for the human body to respond to the influenza vaccine and develop antibodies. Getting the flu shot as close to November as possible, when the flu season typically begins in the U.S., may improve the likelihood of having improved protection during Valentine's day.
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