Is the flu vaccine effective when administered in the summer for the general population in the Northern Hemisphere?

From the Guidelines

The flu vaccine should not be administered in the summer for the general population in the Northern Hemisphere, as it may lead to suboptimal immunity before the end of the influenza season. Ideally, flu vaccination should occur in the fall, typically between September and October, before the flu season begins 1. While summer vaccination will provide protection, the immunity may wane before the peak flu season arrives in winter months. The standard flu vaccine contains four inactivated influenza strains (quadrivalent) that are predicted to circulate in the upcoming season.

• The vaccine takes about two weeks to develop immunity after administration.
• For adults and children over 6 months, a single dose is typically sufficient, while children under 9 receiving the vaccine for the first time need two doses spaced four weeks apart.
• The effectiveness of summer vaccination depends on the individual's immune response and how well the vaccine matches circulating strains.
• Vaccine protection typically lasts 6-8 months, so very early vaccination (June or July) might leave individuals vulnerable during the late winter months when flu activity can still be high. However, if an individual is traveling to an area where influenza is circulating, such as the tropics or the Southern Hemisphere during their influenza season, they should consider receiving the flu vaccine at least 2 weeks before departure 1. In these cases, the benefits of vaccination outweigh the potential risks of waning immunity. It's essential to consult with a healthcare provider to determine the best course of action for individual cases.

From the Research

Effectiveness of Flu Vaccine in Summer

• The flu vaccine is typically recommended to be administered at the beginning of the flu season, which usually starts in October 2.
• However, the Advisory Committee on Immunization Practices (ACIP) recommends that vaccination can occur as soon as the vaccine becomes available, but notes that for nonpregnant adults, vaccination in July and August should be avoided unless there is concern that later vaccination might not be possible 3.
• There is no direct evidence to suggest that the flu vaccine is ineffective when administered in the summer, but the timing of vaccination is important to ensure peak antibody protection during the flu season.
• It takes approximately 14 days after vaccination for a healthy adult to reach peak antibody protection 2.

Considerations for Summer Vaccination

• The flu vaccine composition is updated annually to protect against the most common influenza strains, and it is possible that the vaccine may not provide optimal protection if administered too early 2.
• The ACIP recommends that children who need 2 doses of the flu vaccine should receive their first dose as soon as possible after the vaccine becomes available, to allow the second dose to be received by the end of October 3.
• For pregnant women in the third trimester, vaccination soon after the vaccine becomes available can be considered 3.

General Recommendations

• Routine annual influenza vaccination is recommended for all persons aged ≥6 months who do not have contraindications 3.
• The choice of vaccine should be based on the individual's age and health status, and vaccination should be administered according to the recommended schedule 3.
• There is no evidence to suggest that the flu vaccine is less effective when administered in the summer, but the timing of vaccination is important to ensure optimal protection during the flu season.