The Seasonal Influenza Vaccine

Who should consider getting it; Which vaccine should they get; When should they get vaccinated?

Who should consider getting the seasonal influenza vaccine?

The American Academy of Pediatrics (AAP) recommends annual influenza vaccination of all children without medical contraindications starting at 6 months of age.[1]

The CDC recommends routine annual influenza vaccination is recommended for all persons aged six months and older who do not have a contraindication to vaccination.[2]

Which vaccine should people get?

For children 6 months of age through 18 years[3], unless contraindicated, they may receive any of the following influenza vaccines for their seasonal vaccination:

• Inactivated influenza vaccine – trivalent (IIV3) (egg-based) [Note: Afluria should not be given until age 3 years or after because the manufacturer will only be able to distribute the 0.5 ml pre-filled syringes this year.]
• Inactivated influenza vaccine – trivalent (cell-culture) (ccIIV3) (Flucelvax)
• After age 2 years, live attenuated influenza vaccine – trivalent (LAIV3) (FluMist) may be considered as an alternative to inactivated vaccine, however, do not use if the patient is pregnant or immunocompromised, lives with someone who is pregnant or immunocompromised, is taking aspirin- or salicylate-containing medications, has asthma or has had a history of wheezing in the past 12 months, those without a spleen or with a non-functioning spleen, those with an active leak between the cerebrospinal fluid and the mouth, nose, ear, or other place within the skull, those with cochlear implants, and those who have taken flu antiviral drugs within a certain amount of time (within the past 48 hours for oseltamivir and zanamivir, the past 5 days for peramivir, and the past 17 days for baloxavir).[4]

For adults ages 19 years and older, unless contraindicated, they may receive any of the following influenza vaccines for their seasonal immunization:

• Inactivated influenza vaccine – trivalent (IIV3) (egg-based)[5]
• Inactivated influenza vaccine – trivalent (cell-culture) (ccIIV3) (Flucelvax)[6]
• For those adults 19 – 64 who are immunocompromised by reason of solid organ transplant and immunosuppressive therapy, consider either the trivalent high-dose inactivated influenza vaccine (HD-IIV3) (Fluzone high-dose) or the trivalent adjuvanted inactivated influenza vaccine (aIIV3) (Fluad).
• Adults up through age 49 live attenuated influenza vaccine – trivalent (LAIV3) (FluMist) may be considered as an alternative to inactivated vaccine, however, do not use if the patient is pregnant or immunocompromised, lives with someone who is pregnant or immunocompromised, is taking aspirin- or salicylate-containing medications, has asthma or has had a history of wheezing in the past 12 months, those without a spleen or with a non-functioning spleen, those with an active leak between the cerebrospinal fluid and the mouth, nose, ear, or other place within the skull, those with cochlear implants, and those who have taken flu antiviral drugs within a certain amount of time (within the past 48 hours for oseltamivir and zanamivir, the past 5 days for peramivir, and the past 17 days for baloxavir).[7]

For adults 65 years and older, they should be given one of three options: trivalent high-dose inactivated influenza vaccine (HD-IIV3) (Fluzone high-dose), the trivalent adjuvanted inactivated influenza vaccine (aIIV3) (Fluad) or recombinant influenza vaccine (RIV3) (Flublok).

When should people get their seasonal influenza vaccination?

The CDC states that the optimal time for most people to receive their flu vaccine is during September and October, and while I believe this is good advice for the general public when we aim to educate broadly and not to make things overly complicated, I think that the best advice for those looking to take a more individualized and nuanced approach to ensure the highest protection for the longest period of time of highest transmission is a bit different, as I will explain below.

Special Considerations that would alter the recommended timing of vaccination:

1. Pregnant women: If a pregnant woman will enter her third trimester in July or August, she should consult with her obstetrician or other care giver because it may be best to administer the influenza vaccine then so as to allow for passive transfer of antibodies from the mom to the baby in order to protect the baby as we near the start of the influenza season and the infant will be too young to be vaccinated. Maternal vaccination during the third trimester reduced influenza illness in infants during their first six months of life by slightly more than 50 percent. It takes about two weeks for the mother to make antibodies, so we don’t want to time vaccination too close to the end of pregnancy that the baby might not get the full benefit, especially if delivery came prior to the due date.
2. Children less than 9 years old who will need two doses of influenza vaccine spread out by at least 4 weeks. Given the need to provide the first dose of vaccine at least one month prior to the second dose, it will not be able to time these doses to the initial rise in influenza activity. Therefore, it is reasonable to give the first dose of vaccine when the vaccine first comes out in July or August so that the child can receive the second dose in September or early October before we start to see the rise in influenza cases.
3. International travel. Extensive international travel, or international travel that will involve many potential exposures, or international travel with an extended stays in countries that experience significant influenza activity before we do in the U.S. could be a good reason for early vaccination. This will need to be an individualized decision considering the risks associated with that international travel given where the person is traveling to, the nature of activities that will be engaged in, and the duration of the stay versus the risks associated with expected influenza activity in the U.S. upon the person’s return and their expected exposures upon re-assuming their normal activities.
4. For those people who are seen for an office visit and unlikely to return for a future appointment designed to better optimize the effectiveness of the vaccine, it may be appropriate to go ahead and offer vaccination while the person is there and willing to be vaccinated.
5. For those persons who are very afraid of needles and/or anxious about coming in for shots, but for whom live attenuated influenza vaccine – trivalent (LAIV3) (FluMist) is not a good option, it may be appropriate to go ahead and administer influenza vaccine at the same visit that they are at for another vaccine, such as their updated COVID-19 vaccination, even if the timing is not ideal.

My personal approach to optimizing timing (for those without the special considerations as outlined above).

If we look at the past influenza seasons from the 2014 – 2015 flu season through last year (2024 – 2025) [and throw out 2020 – 2021 because we essentially did not have an influenza epidemic due to our mitigation measures for COVID-19], we find the following (CDC data is recorded by epidemiological weeks, which I have converted to calendar weeks:

                                           Onset                                              peak                                                end

2024 – 2025                   11/24-30                                        1/26-2/1                                         3/23-29

2023 – 2024                   10/29-11/4                                    12/24-30                                        3/24-30

2022 – 2023                   10/2-8                                             11/20-26                                        1/22-28

2021 – 2022                   11/28-12/4                                    12/26-1/1                                      3/16-22

2020 – 2021                   ————–                                      ————-                                        ———–

2019 – 2020                   11/3-9                                             12/22-28                                        4/12-18

2018 – 2019                   12/2-8                                             2/10-16                                           4/7-13

2017 – 2018                   11/26-12/2                                    1/28-2/3                                         4/8-14

2016 – 2017                   12/11-17                                        2/5-11                                             4/2-8

2015 – 2016                   12/13-19                                        3/6-12                                             4/3-9

2014 – 2015                   11/16-22                                        12/28-1/3                                      3/22-28

So, we can see from the above data, that the onset of the seasonal influenza epidemic has ranged from as early as the week of October 2 to as late as the week of December 13, but on average the onset has been around the second to third week of November.

Similarly, the peak influenza activity has occurred as early as the week of November 20 and as late as the week of March 6, but with an average of around the second week of January.

Finally, the annual influenza seasons have ended as early as the week of January 22 and as late as the week of April 12, with an average of around the third week of March.

So, here is how I put it altogether:

1. Influenza vaccine effectiveness (VE) against symptomatic infection can be as low as 14% and as high as 60%[8] [9] (that is pretty good – keep in mind that the vaccine effectiveness against severe disease and death is even higher). That range will depend on factors such as how close a match circulating strains of influenza are to the ones anticipated and included in the vaccines and how close the timing of vaccination is to the rise in influenza activity. Effectiveness is highest in children and young people. That is good because the 2024–25 influenza season had the highest number of pediatric deaths reported (280) since child deaths became nationally notifiable in 2004, except for the 2009–10 influenza A (H1N1)pdm09 pandemic.[10] Approximately one half of children who died from influenza had an underlying medical condition, and 89% were not fully vaccinated. Note that this also means that half of the children who died from influenza did not have an underlying medical condition and would not have been identified as being high risk. Therefore, influenza vaccination provides a significant opportunity to save more children’s lives, especially if we anticipate the right strains for the vaccine and can time the vaccine better.
2. Protection from symptomatic influenza virus infection wanes over the influenza season and therefore, vaccinated individuals are more likely to get infected at the end of the season than at the beginning. I have commonly heard from patients when I have offered them influenza vaccination, “I got the vaccine in the past and caught the flu.” Since the attenuated live vaccine that we have today was not available back then, it was not the vaccine that was giving them the flu. While a 60% reduction in the chance of infection is impressive to me; it is not 100%, so infections still occur. The other possibility is that they simply got vaccinated too early. I have seen pharmacies making the push for influenza vaccines in July and August. Given that vaccine effectiveness for these influenza vaccines wane (decrease) about 9-16% per month, (let’s take a hypothetical VE of 40%. If vaccinated in the middle of July, that person would be at peak effectiveness by the beginning of August. At a loss of 9% in effectiveness each month, the vaccinee would have a VE of 36% in September, 32.4% in October, 29.2% in November, 26.2% in December, and 23.6% in January, which is often when we see influenza activity at its peak. On the other hand, had that vaccination occurred at the end of October – a couple of weeks prior to when we typically see influenza activity on the rise, the VE would be about 32% (vs. 23.6% if vaccinated in July) by the time influenza activity would be expected to peak.
3. Thus, my recommended timing when people ask me is if you just want a date in advance so that you can plan for it, get your vaccine, and you don’t intend to monitor things closely to adjust the timing of vaccination, then plan for the end of October. However, if you are flexible, willing to wait it out, monitor influenza activity and then go in for the vaccine just as influenza activity is beginning to take off in your state or your part of the country, then monitor the wastewater levels of influenza[11] and the outpatient influenza-like illness activity[12] for the evidence that influenza activity is on the rise to get your vaccination. This will give you more protection at the time influenza activity is expected to peak and more protection for further through the flu season.

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[1] https://publications.aap.org/pediatrics/article/doi/10.1542/peds.2025-073620/202845/Recommendations-for-Prevention-and-Control-of?autologincheck=redirected.

[2] https://www.cdc.gov/flu/season/2025-2026.html.

[3] Note that for children under the age of 9 who have not previously received two doses of influenza over their lifetime, they should receive two doses of influenza vaccine this year at least 4 weeks apart. The doses do not need to be the same brand or formulation – a child may receive a combination of IIV, RIV, and LAIV if appropriate for age and health status for this two-dose series. https://publications.aap.org/pediatrics/article/doi/10.1542/peds.2025-073620/202845/Recommendations-for-Prevention-and-Control-of?autologincheck=redirected.

[4] https://www.cdc.gov/flu/vaccine-types/nasalspray.html.

[5] The egg-based vaccines utilize the three following strains for their 2025-2026 vaccine:

• an A/Victoria/4897/2022 (H1N1)pdm09-like virus;
• an A/Croatia/10136RV/2023 (H3N2)-like virus; and (Updated)
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus

[6] The cell culture vaccines utilize the three following strains for their 2025-2026 vaccine:

• an A/Wisconsin/67/2022 (H1N1)pdm09-like virus;
• an A/District of Columbia/27/2023 (H3N2)-like virus; and (Updated)
• a B/Austria/1359417/2021 (B/Victoria lineage)-like virus

[7] https://www.cdc.gov/flu/vaccine-types/nasalspray.html.

[8] https://www.cdc.gov/mmwr/volumes/71/wr/mm7110a1.htm?s_cid=mm7110a1_w.

[9] https://www.cdc.gov/flu-vaccines-work/php/effectiveness-studies/past-seasons-estimates.html?CDC_AAref_Val=https://www.cdc.gov/flu/vaccines-work/past-seasons-estimates.html.

[10] https://www.cdc.gov/mmwr/volumes/74/wr/mm7436a2.htm.

[11] https://www.cdc.gov/nwss/rv/InfluenzaA-national-data.html.

[12] https://www.cdc.gov/fluview/surveillance/usmap.html