Should I still get the influenza A (flu A) vaccine if I have been exposed to flu A?

Should You Still Get the Flu Vaccine After Exposure to Influenza A?

Yes, you should still receive the influenza vaccine even after exposure to influenza A, as the vaccine provides protection against multiple strains and future infections throughout the season. 1

Key Rationale for Post-Exposure Vaccination

The Vaccine Cannot Worsen Your Current Situation

• The influenza vaccine contains only noninfectious viruses and cannot cause influenza infection or worsen any current illness. 2, 1
• Any respiratory symptoms occurring after vaccination represent either coincidental illness unrelated to the vaccine or a pre-existing infection that was already incubating at the time of vaccination. 1

Timing Considerations

• You should wait until you have clinically recovered from acute febrile illness before receiving the vaccine, though minor illnesses with or without fever should not delay vaccination. 2, 1
• There is no required waiting period after influenza infection—only that you have recovered from the acute illness. 1

Why Vaccination Still Provides Benefit

Protection Against Multiple Strains:

• Each year's influenza vaccine contains three virus strains (typically two type A and one type B) representing the viruses likely to circulate during the season. 2, 3
• Exposure to or infection with one influenza A strain does not protect you against other circulating strains, as antibody responses are primarily strain-specific. 3
• Even if you were infected with influenza A(H3N2), you remain susceptible to A(H1N1)pdm09 and influenza B viruses that may be circulating. 4

Limited Duration of Natural Immunity:

• Immunity from natural infection is strain-specific and provides limited protection against antigenically distinct variants. 3
• Influenza A viruses undergo continuous antigenic drift, creating new variants that can evade existing immunity from prior infection. 3

Clinical Effectiveness Data

The vaccine provides substantial protection against severe outcomes:

• 42% effectiveness against influenza-associated hospitalization (overall population). 5
• 52% effectiveness against intensive care unit admission. 5
• 55% effectiveness against need for ventilatory support. 5
• 54% effectiveness against medically attended outpatient illness in persons aged 6 months-64 years. 4
• 71% effectiveness against symptomatic influenza A in children and adolescents. 4

Important Caveats

Common Pitfall to Avoid:

• Do not assume that recent influenza A exposure or infection provides adequate protection for the remainder of the season—the highly mutable nature of influenza A and strain-specific immunity means you remain susceptible to other variants. 3

Vaccine Safety Profile:

• The most common side effect is soreness at the vaccination site (reported by less than one-third of recipients), lasting up to 2 days. 2, 3
• Systemic symptoms (fever, malaise, myalgia) occur infrequently, begin 6-12 hours post-vaccination, and resolve within 1-2 days. 2, 1

Special Considerations:

• Persons with severe egg allergies should consult a physician before vaccination, as vaccines contain small amounts of egg protein. 2, 3
• Pregnant women with high-risk conditions should be vaccinated regardless of pregnancy stage. 2

Bottom Line

Proceed with vaccination once you have recovered from acute illness. The vaccine will protect you against other circulating influenza strains for the remainder of the season, reduce your risk of severe complications, and help prevent transmission to vulnerable contacts. 2, 1 Annual vaccination remains the most effective strategy for preventing influenza and its complications, even in those with recent exposure or infection. 6, 4