A study published in Clinical Infectious Diseases investigated the effectiveness of influenza vaccine throughout the influenza season.
Researchers examined people who were vaccinated with inactivated flu vaccine from Sept.1, 2010, to March 31, 2017, and who were subsequently tested for the flu and respiratory syncytial virus using a polymerase chain reaction test.
They found that people vaccinated 42 to 69 days prior to being tested were 1.32 times more likely to test positive for any strain of the flu as compared with people vaccinated 14 to 41 days prior to being tested.
The odds ratio of testing positive for the flu increased in a linear fashion by approximately 16 percent for each additional 28 days since vaccination. However, there was no evidence of vaccination efficacy waning for respiratory syncytial virus.
"Our results suggest that effectiveness of inactivated influenza vaccine wanes during the course of a single season," study authors concluded. "These results may lead to reconsideration of the optimal timing of seasonal influenza vaccination."