Flu season can start as early as October and last through late May, according to the Centers for Disease Control, which is why many people are lining up to get their shot this month. Some sign up for the vaccine automatically each year, while others are hesitant or avoid it altogether, unsure of whether it’s worth the touted gain. The School of Pharmacy’s Jennifer Kirwin, associate clinical professor in the Department of Pharmacy and Health Systems Sciences, provided a primer on the science behind the vaccine.

Here she offers answers to five commonly asked questions about the flu vaccine.

On how the flu vaccine works

The flu vaccine works by stimulating a person’s immune system to produce antibodies to the influenza viruses expected to be circulating during a given flu season. It takes about two weeks for the antibodies to develop and provide protection from the virus.

On why the vaccine is more effective some years than others

In a given year, vaccine effectiveness varies based on how well “matched” the vaccine is to circulating types, or “strains,” of the flu virus and to the person being vaccinated. The flu vaccine works best for healthy adults and older children, but even for other people, the vaccine can still provide some protection. There are many different strains of influenza virus. Decisions on which strains to include in a given year’s vaccine are based on information about viruses currently circulating worldwide and on predictions of which viruses are most likely to be circulating in the upcoming flu season. It takes several months to produce the vaccine and make it available for use, so decisions about which viruses to include are made very early on. In some years, the vaccine is a better match to the actual circulating viruses than others.

On the difference between the various flu vaccines available

The “trivalent” flu vaccine contains three types of inactivated influenza viruses: two type-A and one type-B. The “quadrivalent” flu vaccine contains the same three types as the trivalent, plus one additional type-B virus. There are both trivalent and quadrivalent flu shots available. One vaccine is not recommended over another when both types are available and safe to use. There is also a live flu vaccine given via nasal mist. The live intranasal flu vaccine is only available as a quadrivalent vaccine.  The viruses in the live vaccine are weakened so they are unable to cause disease. The body uses these inactivated or weakened viruses to stimulate the immune system to produce the antibodies that stop the actual virus in its tracks. Not all people can receive either kind. For example, the intranasal flu mist should only be used in healthy, non-pregnant people between the ages of 2 and 49. Your doctor or pharmacist can tell you more about your options for the flu vaccine.

On the idea that the flu vaccine can “give you the flu”

We hear this quite a bit. There may be a couple of explanations. First, it is possible to develop a mild fever and muscle soreness after receiving a flu vaccine. These mild symptoms may occur because your body is developing the antibodies that will protect you from the actual disease: Your immune system is revving up. While you may not feel 100%, this is not the same as having the actual flu, which can cause serious illness, hospitalization, and, in some cases, death. Also, it takes about two weeks for the vaccine to become effective so it’s possible that someone can contract the flu before he or she is fully protected by the vaccine. A person might also be infected with another non-influenza virus, as the flu vaccine only protects against the influenza virus. Finally, the vaccine is not foolproof. In some cases, vaccinated people may still get the flu, but the illness may be milder than if they were not vaccinated.

On whether or not to get the shot

Influenza can be a serious illness. Risks of serious illness are higher in those older than 65 years, younger than 5 years, and in those with medical conditions that increase their risk of flu-related complications. So, if you are in contact with those types of people, consider vaccinating yourself to help protect them. The vaccine has a good safety record. In adults, the most common side effects of the flu shot are soreness and pain at the injection site, and the most common side effects of the nasal mist are runny nose, sore throat, and nasal congestion. Getting the flu vaccine is also more convenient than ever with vaccines available in most pharmacies, as well as in hospitals and clinics and at area health fairs, such as those offered annually each year at Northeastern. Getting the flu vaccine cannot only prevent you, and the people around you, from contracting influenza, it can also reduce the chance of serious illness if you do get the flu.