This blog was written by the authors of the research article: Lauren Ramsay, Sarah Buchan, Robert Stirling, Benjamin Cowling, Shuo Feng, Jeffrey Kwong, Bryna Warshawsky
In temperate countries influenza circulates for several months each year, affecting approximately 5% of the population. Complications from influenza include pneumonia, ear and sinus infections, worsening of underlying medical conditions, and, on occasion, death. Individuals at higher risk for influenza complications include older adults, young children, people with underlying medical conditions, and pregnant women.
The influenza vaccine generally reduces the chances of influenza infection by about half, thereby decreasing the risk of associated complications
The influenza vaccine is not as effective as other vaccines, and vaccine effectiveness varies from year to year and by influenza strain and age, with effectiveness being somewhat lower in older adults than younger adults. Nonetheless, the influenza vaccine generally reduces the chances of influenza infection by about half, thereby decreasing the risk of associated complications. Influenza vaccination also helps prevent transmission of infection to others.
Vaccination is particularly important for those at higher risk for influenza complications and those able to transmit influenza infection to others (such as health care workers and those in close contact with people at high risk for influenza complications). Some jurisdictions focus influenza vaccination programs on these specific groups, while others offer publicly-funded influenza vaccine to anyone six months of age and over.
Annual vaccination is recommended because the strains in the influenza vaccine can change from year to year, and also because protection from one year may not last until the next. The influenza vaccine contains the three or four strains expected to circulate in the upcoming season. As such, the strains in the Northern Hemisphere vaccine are reviewed each year and updated each February, if needed. Similarly, the influenza vaccine used in the Southern Hemisphere is updated as needed each September.
Repeated influenza vaccinations
Because influenza vaccine is given each year, it is important to understand the impact of receiving repeated influenza vaccinations on the degree of protection offered by the vaccine. The scientific literature has noted some inconsistencies, with some studies suggesting that those repeatedly vaccinated had lower vaccine effectiveness than those vaccinated for the first time or those vaccinated less frequently.
Our recent systematic review of 27 articles and meta-analysis of 20 articles suggested that influenza vaccination in both of two years had similar vaccine effectiveness as vaccination in the current year only. Our study also found that from the patient’s perspective, vaccination in the current year was generally the best option regardless of whether they were vaccinated or not in the prior season.
Our study also found that from the patient’s perspective, vaccination in the current year was generally the best option regardless of whether they were vaccinated or not in the prior season
These findings provide reassurance that for most seasons, influenza vaccine effectiveness will not be substantially lower if the person has been vaccinated in the prior season, although additional research is still needed with regard to the impact of repeated vaccination, especially if given over multiple years.
The impact of repeated vaccination is particularly important for older adults, who are likely to receive numerous influenza vaccinations over the course of their lives. Older adults are at increased risk for influenza complications both because of their age and because of the increasing prevalence of other health conditions that also increase the risk of influenza complications (such as heart disease, lung disease, and diseases that affect the immune system).
Furthermore, older adults living in congregate settings, such as long-term care homes, are at increased risk of acquiring influenza if it is introduced into their living environment. With our aging population, annual influenza vaccination of older adults is also important to mitigate the impact of seasonal influenza on the health care system.
While it remains important to continually study factors that may alter influenza vaccine effectiveness, such as the impact of repeated vaccination, our study, which assessed vaccination over two seasons, provides reassurance that repeated vaccination over this interval generally does not negatively impact influenza vaccine effectiveness.