What’s new with the flu shot?
by Dominic Wu, MD, Harvard Health Blog
November 13, 2017

Should you get the influenza (flu) vaccine this year? The short, quick answer (barring any medical reasons you shouldn’t, such as severe allergies), is yes! But recent research raises another important question: When should you get the shot?

Why a flu shot every year in the first place?
Getting infected with the flu can be dangerous — we’ve seen patients in the ICU who were previously healthy but had a horrible response to a strain of the virus and became very sick.

Every year the Centers for Disease Control and Prevention (CDC) and other agencies release flu vaccination guidelines in late summer to early fall. The flu vaccines are usually available by the end of the summer season. Figuring out exactly which strains of flu viruses the vaccine should protect against is often complicated. Basically, experts look at the influenza virus strains that were making trouble in previous years, and attempt to predict which strains are likely to cause the flu in the upcoming season.

The inactivated flu vaccine is the form of flu vaccine that is most commonly injected and contains parts of the virus but no live virus. Therefore, you should not be infected with the flu from the vaccine itself. Some people may feel “sick” after the vaccine with symptoms such as mild fever, pain in the injection site, fatigue — all of which may just be your body mounting an expected immune response against the foreign virus particles. For the 2017-18 season, the CDC and Advisory Committee on Immunization Practices (ACIP) have recommended against the alternative intranasal live attenuated vaccine, due to concerns about its ineffectiveness during the previous seasons.

Get your flu shot as soon as possible? Maybe not…
Recent studies have suggested that the flu vaccines may not be as effective when given too early. The US Influenza Vaccine Effectiveness Network compiled data on patients seeking care at outpatient clinics during the four previous flu seasons from 2011-12 to 2014-15. Although the data varied for different influenza strains, it did show that the vaccine was most effective at approximately two weeks after it was given. Protection against the flu then decreases every month. There could be many explanations for this: people may lose immunity during the season; different patient populations receive vaccines at different times; the virus may mutate during the course of the flu season, etc. This decrease in protection over time may even account for why some people who did get a flu shot go on to get the flu later on in the season (February or March). However, more research needs to be conducted to evaluate this further.

Comment: I have always been told by my doctors that building maximum immunity after getting the flu shot takes about 4-6 weeks. That corresponds to my personal experience, where if I contracted the flu with a few weeks after getting a flu shot it tended to be more severe than if I got the flu several weeks after the flu shot. I'm not sure about the details of the study mentioned above or why that differs from previous advice and experience. ~ David Baxter

It gets even trickier
The problem is that we cannot predict exactly when the flu season will begin each year. In clinics and in the emergency department, we often see the flu season starting around December and lasting until the spring. And the flu shot is not 100% effective. Over the past few seasons, studies have shown that the vaccine reduced the risk of flu infection by 40% to 60%. For patients who are immunocompromised or have chronic medical conditions, it may still make sense to receive the vaccine earlier so that your body is protected against the virus whenever the season comes. But if you’re generally healthy, it may make sense to get the vaccine a little later in the fall.

But, as physicians, we really aren’t sure yet.

So what should you do?
We need more research to determine the optimal timing of influenza immunization. Until we have more information, I still recommend getting the flu vaccine each year before the flu season starts, especially if you have other health problems, have a weakened immune system, or if your child is young enough to require two doses. Even if you are exposed to the flu virus later in the season and your immunity is not as strong against it, your body may still be able to fight off the virus quicker and more effectively than if you did not receive the vaccine at all. For those who are healthier and have access to primary care physicians, you could consider holding off until later in the fall; however, you may risk getting the flu if the season comes early.

Sources

  1. Intraseason waning of influenza vaccine protection: Evidence from the US Influenza Vaccine Effectiveness Network, 2011-12 through 2014-15. Clinical Infectious Diseases, March 2017.
  2. Seasonal Influenza Vaccine Effectiveness, 2005-2017. Centers for Disease Control and Prevention.