The influenza vaccine, commonly known as the flu shot, is a preventative measure administered each year to protect against the seasonal influenza virus. This annual vaccination cycle is necessary because influenza viruses are constantly changing, requiring scientists to reformulate the vaccine to match the strains predicted to circulate. The overall protective benefit of the flu shot changes annually due to the shifting nature of the virus itself and how well the vaccine components align with the circulating strains.
Understanding Vaccine Effectiveness (VE) Metrics
Evaluating how well the flu shot works involves understanding the distinction between vaccine efficacy and vaccine effectiveness (VE). Vaccine efficacy is determined in controlled clinical trials, calculating the percent reduction in disease among vaccinated individuals compared to a placebo group. Vaccine effectiveness, by contrast, measures the real-world performance of the vaccine in the general population after distribution.
VE is primarily reported in two distinct ways, reflecting different levels of protection. The first metric measures protection against infection or illness, indicating the ability to prevent a person from getting sick at all. The second, and often more robust, metric measures protection against severe outcomes, such as preventing hospitalization, intensive care unit admission, or death. Even when the vaccine is less effective at preventing mild illness, it maintains a significantly higher level of protection against these severe complications.
Current Estimates for the 2024 Season
Initial data for the 2024-2025 Northern Hemisphere season suggests the vaccine offers a moderate level of protection, with a stronger defense against severe disease. Interim estimates for children and adolescents indicate vaccine effectiveness against outpatient illness ranged from 32% to 60%. Protection against more serious outcomes, specifically influenza-associated hospitalization, was notably higher, ranging from 63% to 78% for this age group.
For adults, the effectiveness against medically attended influenza illness was reported between 36% and 54% in outpatient settings. Protection against hospitalization for adults showed a range between 41% and 55%.
The typical effectiveness against all influenza types when the match is considered good falls within a range of 40% to 60%. Early estimates from the Southern Hemisphere, which often informs Northern Hemisphere predictions, suggested a lower effectiveness of about 35% against hospitalization for high-risk groups. This was primarily due to the circulation of specific H3N2 viruses, underscoring the variability that can occur depending on the predominant viral strains.
Why Effectiveness Varies Annually
The need for a new vaccine each year stems from the influenza virus’s continuous and rapid evolution, a process known as antigenic drift. This drift involves minor genetic changes in the surface proteins of the virus, requiring health organizations like the World Health Organization (WHO) to reformulate the vaccine annually. These organizations monitor global influenza activity to select the three or four strains predicted to be dominant in the upcoming season.
The success of the vaccine in a given year is directly tied to how well the selected vaccine strains “match” the strains that actually circulate. When there is a close match, effectiveness tends to be higher, but a poor match can lead to lower protection rates. Strains of the Influenza A(H3N2) virus are historically more difficult to match and often undergo more significant changes, frequently resulting in lower effectiveness during H3N2-dominant seasons.
Individual Factors Influencing Protection
While population-level effectiveness is governed by the virus-vaccine match, an individual’s protection is also influenced by personal biological factors. Age is a significant determinant, with older adults often experiencing a diminished response due to a decline in immune function known as immunosenescence. This age-related change results in reduced production of high-affinity antibodies and less robust T-cell responses.
Individuals with underlying health conditions, such as those who are immunocompromised due to disease or medication, may also mount a lower immune response to the standard vaccine dose. For these high-risk groups, specialized vaccines, such as high-dose or adjuvanted formulations, are sometimes recommended to stimulate a stronger immune reaction.
The timing of vaccination also plays a role because protection can gradually decrease over several months. Studies indicate that vaccine effectiveness in adults can wane by approximately 9% every 28 days, beginning about 41 days post-vaccination. Public health bodies often recommend vaccination in September or October to ensure the highest level of protection is maintained through the typical peak of the flu season in January and February.