Pharmacies and doctors start booking flu vaccine appointments in late August, long before the number of flu cases are on the rise. The CDC recommends getting vaccinated in September or October. But most years, flu cases don’t peak for a couple months. So why September or October?
How long does the flu vaccine last?
First, we need to consider how long the flu vaccine protects you from the flu. If you get the vaccine too early (ex: August), you could be less protected towards the end of the flu season. The CDC says that if possible, avoid getting vaccinated in July or August. On the flip side, if you wait and get the vaccine too late, you could catch the flu before you get the vaccine or before your body has enough time to generate antibodies to provide protection.
Over the years, there have been multiple studies on how long the flu vaccine is effective in different populations. We’ll just highlight a couple here. A study in children aged 2-5 found that October is the optimal time for children to get the flu vaccine. A different study looked at vaccine effectiveness in those 9 years and older using data from the 2011-2012 to the 2014-15 flu seasons. This study found that vaccine effectiveness was highest shortly after vaccination but then declined about 6-11% per month.
The idea is that getting vaccinated in September or October provides some level of immunity throughout the entire flu season.
Referencing historic flu data
Out of curiosity, I’ve been looking at the influenza tracker in my state (Massachusetts) and the CDC national flu tracker. Often these maps and dashboards can overlay this year’s data with previous years’ data so you can see how things track compared to previous years and some overall trends. This can help you get an idea of when to get the flu shot before cases start to rise.
However, historic data is not always predictive of what will happen this flu season. We’re currently in week 38 of the year (at the time of this writing) with most states in the US showing minimal flu activity (exception is DC with moderate flu activity). But go back to the 2021-2022 flu season. This same week 3 years ago showed high flu activity in Texas and DC, with several other states in high to moderate flu activity levels. That year in Massachusetts, the flu peak activity rose much earlier with the peak hospitalizations due to flu in December. (Not surprisingly, the 2020-2021 flu season was minimal because of COVID-19 social distancing.) So if you’re taking a look at historic flu data, it can help you see when flu cases start to rise, but know what happens this flu season might be different than years past.
Consider local rates of flu transmission
Flu transmission rates across regions can differ dramatically. In the subtropics and tropics, influenza transmission occurs year round, sometimes producing multiple peaks in transmission. One study suggests the possibility of a semi-annual vaccine for older adults.
Here in the US, flu season typically runs from October to April, so the CDC advises to get the flu shot in September or October. Of course, the onset of flu season can differ from year to year, which is why it could be helpful to look at current rates of transmission. My state’s dashboard (as well as the CDC dashboards) includes metrics for current influenza transmission with ratings for activity levels (ex: minimal, low, moderate, high), which can be helpful to interpret what the percentages and numbers mean.
Anticipate lag time between vaccination and between protection
While looking at current transmission data is helpful, it takes two weeks after vaccination to generate antibodies against the virus so you’ll have to anticipate what transmission will be like two weeks after you get the vaccine.
For kids receiving their first influenza vaccine, it’ll take a bit longer before you are fully protected. Children who receive the flu vaccine for the first time require two doses spaced at least one month apart so if you plan for your child to be fully protected by November, they’ll have to get their second shot in mid October and their first shot before mid September.
TL;DR
The CDC recommends getting your flu shot in September or October. Check out the CDC’s flu tracker or your state’s flu tracker to see why.
Hope this information is helpful! How are you deciding when to get the vaccine? (I just got mine last Saturday.)
Medical disclaimer: This website does not provide medical advice. It is intended for informational purposes only. No material on this site is intended to be a substitute for professional medical advice, diagnosis, or treatment.
Featured image: Influenza B viral particles. Source: NIAID/NIH.
