When is the best time to get my flu vaccine?

A: Getting a flu vaccination by the end of October is a great way to protect yourself. Although it is available as early as July, getting a vaccine in September or October helps protect you for the part of flu season that typically has the highest rates of infections.

When is flu season? Seasonal influenza, or “the flu”, presents with sudden onset of fever, muscle pain, dry cough, sore throat and runny nose. The flu is caused by infection with an influenza virus. We can see illness from influenza viruses at any time during the year. But like many respiratory viruses, we tend to see the most flu infections in the fall and winter months.

This means the northern hemisphere and the southern hemisphere each have their own flu season. In the US and Canada, we tend to see flu ramping up in October with the peak number of infections occurring between December and February, and then tapering off in May.

Nerd alert! We know this because nerds collaborating with the World Health Organization have been monitoring influenza trends around the world for 70 years! These scientists and epidemiologists collaborate at an annual meeting of the Global Influenza Surveillance and Response System to monitor human influenza infections, look at changes in the virus itself, and plan for seasonal flu vaccinations. Scientists at the WHO collect and analyze this data from around the world all year long to get ready for each flu season. For the Northern Hemisphere, a committee meets in February to put this data together to direct the formula for flu vaccine to be available for the upcoming flu season.

This is a tricky job, because this requires scientists to make predictions of which influenza virus will be circulating most in the coming months and because influenza viruses mutate all the time. In years where there is a good match between circulating influenza virus and the flu vaccine components, it is 40-60% effective at preventing illness. In years where it is not a good match, the effectiveness can be lower. However, even in years where effectiveness is found to be lower, vaccination has other benefits besides preventing illness – studies have shown flu vaccination reduces risk of hospitalization from flu-related illness and reduces the severity of symptoms in people who still get sick.

How long does flu vaccine protection last? Once you take a flu vaccine, it takes about two weeks for antibody levels to increase to peak amount, giving you protection against infection. Those antibodies then start to decrease around 6 months after vaccination, and your immunity and protection starts to decrease too. So most experts suggest timing your vaccine for September or October to allow you to have full protection just ahead of when influenza infections really start increasing and also to keep you protected until the peak of cases likely has passed towards the end of April. If you end up late to the party, there is still definitely benefit in getting a flu vaccine at any point in the flu season.

What if I got my flu vaccine before September – do I need a second one this season? We need more data to say for sure, but probably not. For most individuals, data so far has shown that taking a second dose during flu season is probably safe but has unclear benefits, so it’s not routinely recommended. Some small groups of people, such as those who are immunocompromised due to medications needed to suppress the immune system after an organ transplant, may benefit, but there aren’t strong guidelines on this yet.

Start planning now for your influenza vaccine for this upcoming season. You can reach out to your healthcare provider for guidance or can find a vaccine from many pharmacies at this time of year.

And don’t forget -you can safely take both your seasonal flu vaccine and your bivalent COVID-19 booster at the same time (here’s our post on that topic).