The Science Behind Flu Shots

Although influenza circulates all year, flu virus activity starts increasing in October and spikes between December and February. That means it’s time to start considering how you’ll protect yourself—and the people around you—this flu season.

As with other viruses, you can reduce the chance of getting sick by regularly washing your hands and limiting contact with sick people. But influenza is different from others because it can be potentially life-threatening. During the 2017-2018 flu season, influenza contributed to nearly 80,000 deaths. It’s estimated, however, that 2017-2018’s flu vaccine helped prevent 7 million flu cases, 109,000 flu hospitalizations, and 8,000 flu deaths. The Centers for Disease Control strongly recommends an annual flu vaccine for everyone six months of age and older as the best way to keep yourself and others healthy during flu season. Here’s some more information to help you decide if getting an annual vaccine is right for you.

The injectable form of the influenza vaccine contains what’s called an inactivated virus. Although viruses cannot live, these may also be referred to as “dead” or “killed” viruses because they are non-infectious. Another type of flu vaccine, which usually comes in the form of a nasal spray, contains an attenuated virus. Attenuated means “having been reduced in force or effect,” so these “live,” but weakened flu virus fragments also cannot cause the flu. Instead, inactivated or attenuated viruses cause an immune system response that generates antibodies—blood proteins that target and deactivate specific illness-causing pathogens. Think of vaccines like anti-virus software on your computer; you’re training your system to recognize and defend against problem-causing invaders.

It can take up to two weeks before your body fully develops an immune system defense against influenza after receiving the vaccine. This time window can explain why some people may have still caught the flu shortly after getting vaccinated. It’s not that the vaccine gave them the flu, their immune system simply did not have enough time to develop protection, and they encountered the flu virus. Another possibility is that they became ill from a similar respiratory virus such as rhinovirus, which causes flu-like symptoms. It’s also possible that the strain of flu virus they encountered did not match the viruses used to produce the flu vaccine—this largely contributes to whether a vaccine works well.

Each spring, scientists give their best shot at predicting which flu virus will be most common in the American population during the fall and winter. It’s tricky because viruses mutate from season to season and even throughout one flu season. So, some year’s predictions are more accurate than others. When the Flu Vaccine Effectiveness Network started in 2004, the flu vaccine was a mere 10% effective. But in 2010-2011, the flu vaccine was up to 60% effective. Last year’s flu vaccine was about 40% effective. These fluctuations highlight just how unpredictable the virus can be. But as people continue to get vaccinated, the odds of choosing accurate matches and producing effective flu vaccines increase. If one year’s vaccination is not considered highly effective, studies show a reduced severity of illness in people who get vaccinated but still catch the flu. And even seemingly low levels of efficacy translate to millions of flu cases avoided and thousands of deaths prevented each year.

Once three-to-four of the most likely viruses are determined, they are grown, isolated, purified, and tested. This year, there are both trivalent and quadrivalent vaccines available. Trivalent vaccines contain three different types of inactivated or attenuated viruses. Quadrivalent contain four types. There is no known benefit to receiving one type of vaccine over another. There are currently three approved methods to grow the influenza virus for use in the production of vaccines. Since they all achieve the same result—large amounts of influenza virus—the production method of your flu vaccine is not important.

  • Egg-based flu vaccines contain inactivated or attenuated viruses produced using chicken eggs. In a lab, chosen virus strains are injected into fertilized hen’s eggs and incubated. The protein-rich environment of an egg happens to be the perfect setting for rapid viral growth! After some time, the virus-rich fluid is harvested, inactivated, and purified. The Food and Drug Administration tests and approves vaccines before shipment. Most of this flu season’s vaccinations contain egg-grown influenza, including those used by BWS.
    • According to the Centers for Disease Control, people with an egg allergy can still receive any licensed and age-appropriate egg-based vaccine. It is advised that people with severe egg allergies (any symptom other than hives after egg exposure) should be vaccinated in a medical setting with a health care provider present. The two other production methods listed below contain little to no egg protein and are therefore safer options for people with egg allergies.
  • Cell-based flu vaccines contain influenza grown in mammalian cells. This process may use influenza sourced from egg-based production, but in 2016, the FDA approved cell-grown starters. Like egg-based vaccines, virus-containing fluid can be harvested, inactivated, and purified after incubating for a few days. The Food and Drug Administration then tests and approves vaccines before shipment. Since there is only one FDA approved manufacturer for cell-based flu vaccines, just one of the total eight licensed vaccine options are cell-based this flu season.
  • Recombinant flu vaccine production begins by isolating one gene (hemagglutinin) from one of the recommended virus types. The gene is then combined with portions of a different virus that can grow inside insect cells. The resulting “recombinant” virus is then mixed with insect cells where it can reproduce quickly. After the virus is harvested, the production process is like other vaccine types. There is only one available flu vaccine produced with recombinant technology this year. This production method is 100% egg-free.

Flu vaccine additives—Thimerosal

You may have heard that a flu shot can contain an ingredient called Thimerosal. Thimerosal acts as a preservative to prevent the growth of bacteria and fungus in vaccines. This ingredient is currently only used in multi-dose vials of the flu vaccine where the risk of contamination is significant. It’s included because if bacteria or fungus were to contaminate a multi-use vial of the flu vaccine, multiple people could become sick with a serious infection.

A common concern about Thimerosal is that it contains mercury. However, the form of mercury found in Thimerosal, called ethyl-mercury, can be easily eliminated from the body and does not build-up like methylmercury. Ethyl-mercury is not associated with the health risks of methylmercury.

Although rare, side effects related to Thimerosal in vaccines can include redness and swelling at the vaccine injection site, according to the CDC. Current research does not link Thimerosal to neurodevelopmental disorders. Since 1999, multiple health agencies and vaccine manufacturers agreed that Thimerosal should be reduced or removed from the vaccine supply as a safety precaution, which is why you will only find it in multi-use vials. Most vaccines today do not contain any Thimerosal.

Additional information on the seasonal flu vaccine:

Next Article: Try a New Food: Plant-Based Omegas