For most people who need only 1 dose of influenza vaccine, vaccination should ideally be offered in September and October. For people not vaccinated by the end of October, vaccination efforts should continue as long as influenza viruses are circulating and unexpired vaccine is available.
Vaccination in July and August should be avoided for most groups unless there is concern that vaccination later in the season might not be possible. Early vaccination has been associated with waning of vaccine-induced immunity and decreased vaccine effectiveness before the end of the influenza season, particularly among older adults.
Vaccination in July and August may be considered for people in their third trimester of pregnancy, to allow time for protective maternal antibodies to transfer to the fetus, providing protection during early infancy. Children younger than age 9 years who need two doses of vaccine this season should receive their first dose as soon as possible so that they can get their second dose before the end of October. Children who need only one dose can be considered for vaccination in July or August.