Q+A: How Do Flu Shots Work? And Why Do They Fail?


Entire classes of students calling out sick. Young, healthy adults dying. Hospitals overwhelmed with too many patients and not enough IV bags.

Illness is plaguing thousands of Americans during what officials say is the worst flu season in nearly a decade.

Doctors are urging everyone: Get your flu shot.

While the message seems to be sticking, questions about the vaccine’s effectiveness — and perceived side effects — have held some people back from protecting themselves against this deadly epidemic.

Jennifer Hamilton, MD, PhD, an associate professor and primary care doctor at Drexel’s College of Medicine says that while the vaccine may not be 100 percent protective against this year’s particularly potent strain of flu, it’s still your best defense.

How does the flu vaccine work?
There are several different ways to make the flu vaccine. Most come from inactivated or “killed” viruses (the flu shot), and some are made from the live, weakened virus (nasal drops). Whichever way they’re made, the vaccines work by showing the body what the flu virus is like before an actual infection occurs. This lets the immune system respond to a real flu virus more quickly, ideally getting rid of the virus before it can cause a generalized infection.

How effective is the flu vaccine? Why is it not 100 percent protective?
The main reason that the flu vaccine is not 100 percent protective is because the flu virus is always changing. It takes months for the vaccine to be produced and distributed. While that’s happening, wild influenza is changing as it spreads from person to person. The more changes happen, the less effective the vaccine is.

Making matters trickier, it’s technically difficult to make a good vaccine for this year’s main strain of flu, H3N2. Even though most flu vaccines don’t have any live virus in them, most start with live virus at some point in the process. Viruses need to grow in cells. Historically, we’ve used chicken eggs to culture the virus. But the H3N2 that comes out of a chicken egg is shaped just a little different than the H3N2 that’s spread from human to human. So, the vaccine doesn’t work as well as we’d like.

The good news is that even though the vaccine isn’t 100 percent effective, people who get the flu after getting the vaccine usually experience a milder infection. It’s not as likely to send someone to the hospital, for example.

Some people claim that the vaccine actually caused them to become infected with the flu. Is this possible?
You can’t get the flu from the flu shot, because no shots have live flu virus. Even the vaccines that have live virus (these ones are given as drops in the nose) are weakened to the point that they don’t cause flu.

With that said, some people do feel sick after getting the vaccine. The vaccine works by getting your body prepared to fight off an infection. A lot of those immune-system responses in and of themselves can make people feel a little under the weather for a day, like the feeling you get when you’re about to get a cold. The flu is a lot worse!

Another reason for getting the flu after getting the vaccine is that some people wait until flu is widespread to get the vaccine, and already have the infection starting by the time they get vaccinated. It takes about two weeks for the vaccination to have its full effect.

How can young, healthy people die from the flu?
Flu is a dangerous infection by itself, plus it can make you more vulnerable to other infections. Most people who die from the flu get another illness on top of it; pneumonia is the most common. Other times, young healthy people can have such a strong response to the flu that the immune system itself becomes part of the problem, causing swelling in the lungs and making it hard to breathe.

The fact that young people can die from the flu is part of the reason that the CDC recommends the flu vaccine even for infants as young as six months old. The vaccine isn’t just for older people, or people with chronic health conditions. It’s for almost everyone.

When should someone go to the doctor if they are feeling flu symptoms?
Flu symptoms come on suddenly. Fever, chills and muscle aches can help you tell the flu from a cold. If you have those symptoms — especially if the fever is over 101.5 — call your doctor. Right now, many healthcare offices would rather have you call than come right in. They can give you some advice about what to do next over the phone.

Is it too late to get the vaccine if you have not already?
No, it’s not too late. There’s no way to tell how long the flu season is going to last.  The vaccine isn’t perfect, but it’s one of the best tools we have for preventing the flu. If you do wind up getting the flu anyway, it’s likely to be less severe.  Other ways to protect yourself include frequent hand washing and staying away from crowds. If you feel sick yourself, stay home to avoid spreading it to other people.

For media inquiries, contact Lauren Ingeno at lingeno@drexel.edu or 215.895.2614.


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