Can Receiving a Flu Shot Each Year Actually Weaken its Effectiveness?

Flu activity has continued to rise in the United States this past week, with health departments across the country reporting widespread influenza epidemic conditions.

The Centers for Disease Control and Prevention’s (CDC) weekly FluView report notes that the proportion of individuals who have visited their healthcare providers with symptoms of influenza-like illness rose to 4.8% during the week ending February 4, a number that is up from the previous week’s rate of 3.9%; this number is also above the national baseline of 2.2% for the eighth week in a row.

Since the start of the 2016-2017 flu season, the CDC has reported 6,804 laboratory-confirmed cases of influenza-related hospitalizations throughout the country. In Oklahoma, one of the 43 states that have been hit particularly hard by widespread flu activity, health officials have now reported a total of 827 hospitalizations and 23 deaths this season, with the highest rate of hospitalization among adults aged 65 and older.

While the CDC does not track the flu-related adult mortality rates each season, weekly flu reports do monitor pediatric flu deaths. The CDC has reported five new cases of childhood deaths associated with influenza infections, bringing the total up to 20 children for the season. One such case occurred in North Carolina’s Pitt County, where officials say a child with an existing health condition died of complications from the flu, though no additional details about the child have emerged. Another death involved a 10-year-old girl in Pennsylvania’s Washington County, near Pittsburg. The girl was admitted to a local hospital after experiencing flu symptoms; it is unclear whether she had received a flu shot this season.

Health officials around the country continue to emphasize that it’s not too late to receive a flu shot this season to help protect against infection. However, as news of more flu-related illnesses and deaths are reported, a new study published in The Journal of Infectious Diseases has revealed that the flu shot may be less effective for those who receive the shot two years in a row. The study, conducted by Canadian researchers and lead by the British Columbia Centre for Disease Control in Vancouver, found that individuals vaccinated for prior flu seasons may receive less protection from the flu shot in the following year and experience reduced vaccine effectiveness due to what the study team called “antigenic distance hypothesis.”

In the late 1990s researchers came up the antigenic distance hypothesis, a hypothesis that aimed “to reconcile variable observations of repeat influenza vaccination effects.” The researchers developed a mathematical model to measure vaccine effectiveness; however, that model had no “absolute clinical meaning,” according to the study. Using this model, the researchers studied how antigenic distance between a flu vaccine administered in the prior flu season and one administered for the same strain in the current season changed the vaccine’s effectiveness. They found that when there is low antigenic distance, or little difference, between vaccine components for consecutive flu seasons, the vaccine from a previous season can create negative interference during the next season. The researchers attribute this to the “underlying theory of associated memory” which states that the former vaccine’s antibodies may interfere with those of the subsequent vaccine antigen, and that the second vaccine’s “stimulation of rapid [vaccine 1] memory responses potentially protective against the epidemic strain. The findings suggest that serial vaccination may be why some individuals get the flu even though they’ve received the correct flu shot that matches the circulating strains.

Researchers are working on developing universal long-lasting flu shots that would be administered every 3 to 5 years and would offer effective protection against the influenza virus. While the flu shot varies in effectiveness from season to season, when the flu vaccine matches closely with circulating strains, the CDC notes that the shot is the best way to reduce the risk of catching the flu and when more individuals are vaccinated the virus is less likely to spread and cause outbreaks.