How effective is the flu shot?

Last week, I discussed the side effects of the flu vaccine, which can range from anything mildly uncomfortable to more serious illnesses. In that post, I also mentioned which groups of people are considered “high risk” for flu-related complications and should definitely be vaccinated, including babies, pregnant women, and seniors.This week, let us delve into the effectiveness of the flu vaccine in fighting influenza.

According to the CDC, how well the shot works, or the vaccine’s “efficacy,” depends on a few different factors. The two main influences, however, are the characteristics of the person being vaccinated and how well “matched” the vaccine is to that particular strain of the flu.

First and foremost, the vaccine’s efficacy relies on the age and health status of the recipient. By studying polls taken after every flu season, doctors and scientists at the CDC estimate that the flu shot is effective in preventing this illness in about 60% of healthy individuals under the age of 65 years.

The graphs shown below were made after the 1982-1983 flu outbreak in Genesee County, Michigan. By comparing the incidence rate of infection amongst vaccinated and unvaccinated Nursing Home residents, the results suggest that the flu shot was largely effective in immunizing healthy adults surrounded by sick patients.

Graph Attribution

Although they also say that the flu shot is less effective in older individuals, the CDC holds that this vaccine reduces complications related to the flu and other common illnesses. Some studies show that the shot was 50-60% effective in reducing hospitalization and 80% effective in lowering the death rate amongst older recipients.

Now, why would the vaccine have a different effect on older individuals?

Human immune defenses naturally become weaker with age. With this in mind, the standard flu shot can actually be risky for older people because they may have a diminished immune response and might not produce as many antibodies as they could have in the past.

This is why a different kind of vaccination is given to people over 65 years old. Called the Fluzone High-Dose influenza vaccine, this injection contains four times the amount of antigen, otherwise known as the substance that causes your body to produce antibodies, than the standard flu shot administered to younger people. This is intended to create a stronger immune response in the recipient, and would explain why the flu shot is so useful in preventing complications related to the flu virus.

One question remains: Does this special shot actually bring about a better immune response?

According to data from clinical trials of older recipients, it definitely results in levels of antibodies being produced. Whether or not these antibodies actually reduce the chances of getting the flu virus, however, is subject to ongoing debate. According to a study in the New England Journal of Medicine, though, the high dose vaccine was 24.2% more effective in preventing the flu amongst older individuals, so scientists are confident that the higher antigen dose is a good idea.

After having explored how age and health status influence the effectiveness of the flu vaccine, let us discuss the second factor that I mentioned before: the relationship between the shot and the strain of virus it is fighting.

Earlier in this post I mentioned how well “matched” the vaccine and the illness are. What did I mean by this?

Well, the vaccine has proven to be most effective when its antigens match those of circulating strains. If they match, then the pathogens introduced to the body by the vaccine will bring about an initial production of antibodies that will successfully fight off the current flu virus.

That being said, it is very difficult to predict which strain of the virus will break out each year and created a corresponding flu vaccine, so sometimes the vaccinations do not “match” and are less effective at fighting the illness.

This exact situation occurred during the 2014-2015 flu season when the CDC released a warning that the flu season would be particularly unpredictable and harsh due to an outbreak of the H3N2 virus.

According to CDC officials, a vaccine was made and ready to go until the virus “drifted” in March, a mutation that came too late to be included in the vaccine.

"It's clear that the laboratory tests we're using to understand how closely related this virus is to the vaccine virus are good, but they're imperfect markers of how well the vaccine will function in the field," said Joseph Bresee, chief of the CDC's Influenza Epidemiology and Prevention Branch.

Although this seems scary for recipients of the flu vaccine, we can rest assured that, even when the shot is not perfectly “matched” to the strain, they still provide some protection against the harsher effects and complications of the flu virus.