Every August and September, the flu shot signs start appearing at pharmacies and pediatric offices, and
parents often ask us the same question:
Should I get my child vaccinated now, or should I wait until later in the fall?
The short answer: For most children who need only one flu vaccine this season, September or October is a great time to get it. But there are some important exceptions, particularly for younger children who need two doses.
Why not just get the flu shot as early as possible?
It takes about two weeks after vaccination for the immune system to build its full response. But that
protection doesn’t remain at exactly the same level throughout the year.
Studies looking at antibody levels after influenza vaccination have found that antibody levels peak after
vaccination and then gradually decline. In one study, antibody levels had decreased by about three months after vaccination, although measurable antibodies persisted for at least six months.
That doesn’t mean the vaccine suddenly “stops working” at three or six months. Immunity is much more
complicated than a single antibody measurement. But studies of real-world vaccine effectiveness have also found some decrease in protection as the months pass.
And that creates a bit of a timing puzzle.
We’re trying to hit a moving target
Flu season doesn’t follow a calendar perfectly.
Influenza can begin circulating in the fall, but peak flu activity may not occur until January, February, or
even later. We can’t know in September exactly what this year’s flu season will look like.
Vaccinate very early, and protection may have waned somewhat by the time influenza peaks.
Wait too long, and your child could be exposed before they’re protected.
The goal is therefore not to find one magical vaccination date. It’s to time vaccination so that children are protected before flu begins circulating widely while maintaining as much protection as possible
through the winter.
For most families, that makes September and October the sweet spot.
So when should my child get vaccinated?
For most children who need just one flu vaccine this year, September or October is a good target, ideally
completing vaccination by the end of October.
There is no reason to panic if your child hasn’t received a flu vaccine by Halloween, though. Vaccination can still be valuable in November, December, and later, as long as influenza continues to circulate.
There is also more flexibility for kids. Unlike older adults, for whom waning immunity is a particular
concern, the evidence for clinically important waning in children is less clear. If your child’s annual checkup happens in August and you aren’t sure you’ll make it back to the office in the fall, getting the vaccine then may make more sense than risking not getting it at all.
The best-timed flu shot isn’t very helpful if it never gets given.
The big exception: children who need TWO doses
Children between 6 months and 8 years old, who have never been vaccinated before, need two doses of
influenza vaccine during the season, separated by at least four weeks.
For these children, start early.
The first dose can be given as soon as that season’s vaccine becomes available, even in July or August.
Starting early gives enough time for the second dose to be administered at least four weeks later, ideally
allowing the series to be completed by the end of October.
If you’re not sure whether your child needs one dose or two, ask your pediatrician. It depends on your
child’s previous influenza vaccination history.
The bottom line
Think of flu vaccination as a balancing act.
We want to vaccinate early enough that children are protected before influenza arrives, but not
unnecessarily early when waiting a few weeks may provide stronger protection later in the season.
For most children who need one dose:
Aim for September or October.
For children who need two doses:
Start as soon as the vaccine is available so there is time to complete both doses. And if it’s November,
December, or even later and your child still hasn’t received their flu vaccine? It’s not too late. Get it.
Flu vaccination isn’t perfect, and its effectiveness varies from year to year. But vaccination continues to
reduce the risk of influenza and, importantly, can reduce the risk of serious illness, emergency visits, and
hospitalization in children.
Sources
Centers for Disease Control and Prevention/Advisory Committee on Immunization Practices. Prevention and Control of Seasonal Influenza With Vaccines.
American Academy of Pediatrics Committee on Infectious Diseases. Recommendations for Prevention and Control of Influenza in Children, 2025–2026.
Mordant FL, Price OH, Rudraraju R, et al. Antibody titres elicited by the 2018 seasonal inactivated influenza vaccine decline by 3 months post-vaccination but persist for at least 6 months. Influenza and Other Respiratory Viruses. 2023.
Doyon-Plourde P, Przepiorkowski J, Young K, Zhao L, Sinilaite A. Intraseasonal Waning Immunity of Seasonal Influenza Vaccine: A Systematic Review and Meta-Analysis. Vaccine. 2023.
Ferdinands JM, Fry AM, Reynolds S, et al. Intraseason Waning of Influenza Vaccine Protection: Evidence From the US Influenza Vaccine Effectiveness Network. Clinical Infectious Diseases. 2017.
Tenforde MW, Reeves EL, Weber ZA, et al. Influenza vaccine effectiveness against hospitalizations and
emergency department or urgent care encounters for children, adolescents, and adults during the 2023–2024 season, United States. Clinical Infectious Diseases. 2025.
