You need a flu shot once every year. Unlike childhood vaccines that provide long-lasting immunity, the flu vaccine is designed for a single season of protection. Two factors make annual vaccination necessary: the flu virus mutates constantly, so the vaccine formula changes each year to match circulating strains, and your immune response from the shot fades over the months following vaccination.
Why the Flu Shot Is Annual
Flu viruses change their surface proteins through a process called antigenic drift, making last year’s vaccine a poor match for this year’s virus. The World Health Organization meets twice a year to analyze which strains are circulating and selects the best candidates for the upcoming vaccine. The Northern Hemisphere formula is chosen each February, giving manufacturers time to produce updated vaccines before fall.
Even if the strains didn’t change, a single flu shot wouldn’t carry you through multiple seasons. Protection peaks about two to four weeks after vaccination, then starts declining. A systematic review published in The Journal of Infectious Diseases found that vaccine effectiveness drops significantly within three to six months, particularly against H3N2 strains (the type that tends to cause the most severe seasons). One reanalysis of a randomized trial found that efficacy started near 100% right after vaccination but fell to roughly 50% after just 12 weeks. Other research puts the decline at approximately 8 to 9% per month in the general adult population, and 10 to 11% per month in adults 65 and older.
Best Time to Get Vaccinated
September and October are the sweet spot for most people. Flu activity in the U.S. typically picks up in November, peaks between December and February, and can linger into March or April. Getting your shot in September or October means your immunity is strongest during the months you’re most likely to encounter the virus.
Getting vaccinated too early can leave you less protected later in the season. If you get your shot in July and flu peaks in February, that’s seven months of waning immunity. For this reason, adults 65 and older and people in early pregnancy are specifically advised to avoid July and August vaccination unless there’s a real chance they won’t be able to come back later. That said, getting vaccinated late is still better than skipping it entirely. Vaccination is recommended as long as flu viruses are still circulating, even into January or beyond.
Children May Need Two Doses
Most adults and older children need one dose per season, but young children sometimes need two. Children aged 6 months through 8 years need two doses, spaced at least four weeks apart, if they’ve never received a flu vaccine before or haven’t previously gotten at least two doses in their lifetime. This is because young children who are new to both the vaccine and the virus itself don’t mount a strong enough immune response from a single shot. Research from the Center for Infectious Disease Research and Policy confirms that a second dose provides meaningful additional protection for vaccine-naive children, especially those under 3.
Once a child has received at least two lifetime doses (even across different seasons), they only need one dose per year going forward, just like adults. If you’re unsure of your child’s vaccination history, the default recommendation is to give two doses.
What About Getting a Second Shot Later in the Season?
Some people wonder whether a booster midseason would help, especially if they got vaccinated early or belong to a high-risk group. Current guidelines do not recommend a second dose for adults within the same season. While the logic of boosting waning immunity makes sense on paper, there isn’t enough clinical evidence yet to support the practice as a routine recommendation. The better strategy is to time your single dose well, aiming for September or October so that peak protection lines up with peak flu activity.
Stronger Options for Older Adults
Adults 65 and older face the highest risk of serious flu complications, and their immune systems also produce a weaker response to standard vaccines. Three enhanced vaccine options are available for this age group: a high-dose version, an adjuvanted version (which contains an ingredient that stimulates a stronger immune response), and a recombinant version made with a higher concentration of the active component. Any of these is preferred over the standard-dose shot for people 65 and older, though a standard dose is still recommended if the enhanced versions aren’t available.
Timing matters even more for this group. Because immunity wanes faster in older adults, getting vaccinated in late September or October rather than midsummer helps ensure better protection through the peak months. The goal is to balance early enough protection against the risk of fading immunity before flu season ends.