Doctor, researcher or scientist hand in blue glove holding flu vaccination medicine vial dos

Answer

This is a great question and, unfortunately, the answer is a bit muddled.

To start, a number of clinical studies report that the effectiveness of the flu vaccine does decrease over time during a single flu season.

For example, one study, published in Clinical Infectious Diseases, concluded the following:

"We observed decreasing influenza vaccine protection with increasing time since vaccination across influenza types/subtypes. This association is consistent with intraseason waning of host immunity, but bias or residual confounding could explain these findings."

This particular study used data collected from flu seasons 2011-2012 through 2014- 2015 and broke down infection rates by strain.

They found that vaccine effectiveness decreased by about 7% to 11% per month after vaccination for influenza A (strains H3N2 & H1N1) and influenza B.

Other studies have found similar results. The following is the conclusion from a study published in the same journal (Clinical Infectious Diseases) in May 2019:

"Our results suggest that effectiveness of inactivated influenza vaccine wanes during the course of a single season. These results may lead to reconsideration of the optimal timing of seasonal influenza vaccination."

Authors of this study found that the effectiveness of the flu vaccine decreases 'approximately 16%' for every 28 days after vaccination.

So, based on the results of these studies (and more), it naturally brings up concern as to whether or not you can get it too early.

Adding to this, it seems that each and every year, the flu vaccine becomes available at your local pharmacy earlier and earlier (sometimes even as early as July and August!).

The next section looks at flu vaccine timing in more detail and discusses what the experts recommend.


Timing Recommendation

The CDC (Centers for Disease Control) and ACIP (Advisory Committee on Immunization Practices) acknowledge that the effectiveness of the flu vaccine may decrease over time:

"For those requiring only 1 dose for the season, early vaccination (i.e., in July and August) is likely to be associated with suboptimal immunity before the end of the influenza season, particularly among older adults."

Nevertheless, there is not a recommendation against getting the vaccine as soon as it is available. They only explicitly recommend getting the vaccine by the end of October (as well as after that if you have yet to receive it, but by the end of October is optimal).

The guidance they (the CDC and ACIP) provide for health care providers is to make the vaccine available to patients as soon as it is released...this is for a number of reasons.

Primarily, there is a concern that defining an 'ideal flu vaccination period' would decrease overall vaccination rates (or 'opportunities' as the CDC puts it). A patient at an August doctor visit for example, may not come back for their flu shot in another month or two and should be given the vaccine in August if available.

Additionally, it is extremely difficult to accurately predict when flu season actually starts, since it varies yearly.

The CDC sums it up as follows:

"Variable data concerning presence and rate of waning immunity after influenza vaccination, coupled with the unpredictable timing of the influenza season each year, prevent determination of an optimal time to vaccinate. Programmatic issues are also a consideration; although delaying vaccination might result in greater immunity later in the season, deferral also might result in missed opportunities to vaccinate."

Adding to this uncertainty as to the optimal time to vaccinate, some studies, which use mathematical models to predict how vaccination times could potentially affect certain complications (e.g. hospitalizations), report that not getting the vaccine when it becomes available can have negative consequences.

One such study predicted if more than 14% of older adults that usually get vaccinated in August and September failed to get vaccinated until October, there would be a significant increase in hospitalization rates due to flu complications early in the season (when it often is at its worst).


Final Words

The overall point is that there are a lot of factors at play, and there is no definitive answer regarding the best time to get vaccinated (if there even is one).

Even though it may be better to wait until October to get vaccinated for some individuals, it's better to get it early than not at all. Additionally, it is possible flu season may start unpredictably early any given year.

As a side note, I want to point out that everything written above pertains to those over 9 years old and over.

Some children aged between 6 months through 8 years may require 2 doses of the flu vaccine (e.g. those who have never been vaccinated against influenza).

In this population, it is recommended to receive the first dose as soon as possible and the second dose by the end of October (doses must be separated by at least 4 weeks).


Summary

  • A number of studies report that the efficacy of the flu vaccine decreases over time (within a single season).
  • The CDC (Centers for Disease Control) and ACIP (Advisory Committee on Immunization Practices) do acknowlege that effectiveness may decrease over time, but that must be weighed against missed vaccination opportunities and other factors (e.g. early flu seasons).
  • Overall, the recommendation by the CDC is to get the flu vaccine by the end of October, but it is still beneficial to receive it before (or after) that time if necessary.

  • References
    1. Waning of influenza vaccine protection: Exploring the trade-offs of changes in vaccination timing among older adults. PubMed
    2. Key Facts About Seasonal Flu Vaccine. CDC
    3. Intraseason Waning of Influenza Vaccine Protection: Evidence From the US Influenza Vaccine Effectiveness Network, 2011–2012 Through 2014–2015. Oxford Academic
    4. Waning vaccine protection against influenza A (H3N2) illness in children and older adults during a single season. ScienceDirect
    5. Is It Possible to Get a Flu Shot Too Early? JAMA Network
    6. Intraseason Waning of Influenza Vaccine Protection: Evidence From the US Influenza Vaccine Effectiveness Network, 2011–2012 Through 2014–2015. Oxford Academic
    7. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2019–20 Influenza Season. CDC
    8. Seasonal Influenza Vaccine Dosage & Administration Questions & Answers. CDC