September and October are generally good times to be vaccinated. Flu activity often begins to increase in October and November and peaks between December and February. Adults, especially those older than 65, should not get vaccinated early (in July or August) because protection in this group may decrease over time. Children can get vaccinated as soon as vaccine becomes available. You should not wait for flu activity to be rising or high in your community to get a flu vaccine. It takes about two weeks after vaccination for antibodies to develop in the body that protect against flu.

Multiple studies conducted over different seasons and across flu vaccine types and influenza virus subtypes have shown that the body’s immunity to influenza viruses (acquired either through natural infection or vaccination) declines over time. The decline in antibodies is influenced by several factors, including the antigen used in the vaccine, the age of the person being vaccinated, and the person’s general health.  Getting vaccinated each year provides the best protection against the flu throughout flu season. It is important to get a flu vaccine every season, even if you got vaccinated the season before and the viruses in the flu vaccine have not changed for the current season.

There is no preferential recommendation about getting the flu and COVID vaccines at the same time versus spreading them out. You can receive both vaccines at the same time, and if you spread them out there is no recommended waiting time between getting them. A CDC study showed people who received both vaccines at the same time were slightly more likely (8-11%) to have side effects, but they were generally mild and resolved quickly.

All flu vaccines for the 2025-26 influenza season will be trivalent vaccines, which cover the 3 most common strains of influenza. Below are the 2025-26 trivalent influenza strains:

Egg-based influenza vaccines:

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

Cell- or recombinant-based influenza vaccines:

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

In 2022, the CDC adopted the Advisory Committee on Immunization Practices’ (ACIP) recommendation to preferentially recommend the use of specific flu vaccines for adults 65 years and older, including higher dose and adjuvanted flu vaccines. The preference applies to Fluzone High-Dose, Flublok and Fluad flu vaccines. This recommendation was based on a review of available studies which suggests that, in this age group, these vaccines are potentially more effective than standard dose unadjuvanted flu vaccines. 

There are two different flu vaccines for adults 65 and older. Both are designed to give older adults a better immune response to protect against the flu. One type is a high-dose vaccine that contains four times the amount of antigen (active ingredient) as the regular-dose flu shot. The second type contains a standard dose with an adjuvant which helps enhance the immune system’s response.

People with an egg-allergy may receive any flu vaccine, regardless of whether it is egg-based or non-egg based. The CDC adopted the 2023-2024 Advisory Committee on Immunization Practices’ (ACIP) recommendation that the previous additional safety measures for individuals with egg-based allergies are no longer recommended.

You can receive the flu vaccine if you have mild illness symptoms, like a cough. Individuals with moderate to severe illness should wait to be vaccinated until their symptoms resolve.

The flu vaccine cannot cause the flu since it contains inactivated viruses. Less than 1% of people experience side effects such as a low-grade fever or muscle aches, which can be mistaken for the flu. The flu vaccine is not fully effective until 2 weeks after getting the shot. Unfortunately, you may get the flu even if you get the vaccine, but this is from exposure to a sick person, not the vaccine itself.

Yes, you should still get the flu vaccine if you already had the flu this year. There are many types of flu virus. Infection from one virus type does not protect you from other virus types. The flu vaccine can help protect you from 3 virus types.

Multiple studies conducted over different seasons and across flu vaccine types and influenza virus subtypes have shown that the body’s immunity to influenza viruses (acquired either through natural infection or vaccination) declines over time. The decline in antibodies is influenced by several factors, including the antigen used in the vaccine, the age of the person being vaccinated, and the person’s general health.  Getting vaccinated each year provides the best protection against the flu throughout flu season. It is important to get a flu vaccine every season, even if you got vaccinated the season before and the viruses in the flu vaccine have not changed for the current season.

Some people can still become infected with a flu virus, despite getting vaccinated. Protection provided by flu vaccination can vary widely, based in part on health and age of the person getting vaccinated. In general, the flu vaccine works best among healthy younger adults and older children. Flu vaccination may make your illness milder if you do get sick. It is possible to get flu after vaccination due to an exposure before your body has built a full immune response (2 weeks) or exposure to a strain not included in the seasonal flu vaccine.

Influenza vaccine effectiveness can also vary seasonally and among different age and risk groups and even by vaccine type. How well the vaccine works can depend in part on the match between the vaccine viruses used to produce vaccine and circulating viruses that season. It is not possible to predict which flu viruses will predominate. The CDC monitors circulating viruses throughout the year and provides information about their similarity to the flu vaccine viruses as it becomes available. Final estimates are not available until after the season is over. A less-than-ideal match may result in reduced vaccine effectiveness against circulating flu strains, but it can still provide some protection against flu illness.

Thimerosal is used in vaccines to prevent potentially life-threatening contamination with harmful germs. There is no scientific evidence that thimerosal in vaccines causes harm except in those who have an allergy to thimerosal.  On July 23^rd, 2025, the US Health & Human Services adopted the removal of thimerosal use in vaccines.