What are the consequences of not receiving the influenza (flu) vaccine by summer?

From the Guidelines

It is crucial to get the flu vaccine as soon as it becomes available, ideally by the end of October, to ensure optimal protection against influenza viruses. The timing of influenza vaccination is important, as influenza viruses can begin to circulate in early fall and continue to late spring, with peak activity typically occurring from January through March in the United States 1. Although summer months usually have minimal flu activity, influenza cases and outbreaks can still occur, particularly among international travelers who may be exposed to influenza year-round, depending on destination 1.

When considering the duration of protection, studies suggest that immunity after influenza vaccination can wane over time, especially in adults, with very early vaccination (July or August) potentially associated with suboptimal immunity before the end of the influenza season 1. However, there is less evidence of waning immunity in children, and some studies have found that antibody levels remain elevated compared to pre-vaccination levels for 6 months after vaccination 1.

Key points to consider:

• The American Academy of Pediatrics (AAP) and the Centers for Disease Control and Prevention (CDC) recommend that children, especially those who need 2 doses, should be immunized as soon as a vaccine becomes available and complete influenza vaccination by the end of October 1.
• Individuals who did not receive the current seasonal influenza vaccine during the Northern Hemisphere fall and winter season and are traveling to parts of the world where influenza activity is ongoing should consider seasonal influenza vaccination 2 weeks before departure, if available 1.
• Practices should continue to vaccinate individuals as long as influenza viruses are circulating and an unexpired vaccine is available 1.

In terms of real-life clinical practice, getting vaccinated as soon as possible after a vaccine becomes available is the best approach to ensure optimal protection against influenza viruses, especially for high-risk groups such as young children, elderly individuals, or those with chronic health conditions 1.

From the Research

Flu Vaccine Timing

• The flu vaccine is typically recommended to be administered at the beginning of flu season, which usually starts in October 2.
• It takes approximately 14 days after vaccination for a healthy adult to reach peak antibody protection 2.
• If the flu vaccine is not administered by summer, it is still important to get vaccinated as soon as possible to ensure protection against the flu virus.

Consequences of Delayed Vaccination

• Delaying vaccination may leave individuals unprotected against the flu virus during the early months of the flu season 2.
• The flu virus can undergo antigenic drift, making the vaccine less effective in some seasons, which highlights the importance of annual vaccination 2.

Alternative Prevention Measures

• Antiviral medications, such as oseltamivir, can be used as a preventive measure against the flu virus, especially for individuals who are at high risk or have not been vaccinated 3, 4.
• Oseltamivir has been shown to be effective in preventing symptomatic laboratory-confirmed influenza in healthy adults and at-risk elderly subjects 4.

Importance of Annual Vaccination

• Annual vaccination is necessary due to the constant evolution of influenza viruses 5.
• While repeat vaccination may result in a reduction in vaccine effectiveness in subsequent years, the available evidence suggests that annual vaccination is still the best way to protect against the flu virus 5.