What You Need to Know about Flu Shots

It’s flu shot season, but even people who support childhood vaccines are sometimes a bit iffy about the flu shot. Is it really necessary, or just a scam? Can you get sick from it? Do you really have to get it every year? Let’s try to put your mind at ease.

Is the flu even that serious?

Yes, it can be. For most of us, the flu is just an inconvenience, so we forget that it can kill people. A study in 2010 found that between the 1976-77 and 2006-7 flu seasons in the U.S., at least 3,000 people died of the flu each year and in the deadliest flu season, there were nearly 49,000 flu-related deaths. Most fatalities were in adults age 65 and older. That doesn’t mean younger people should skip the shot; herd immunity helps protect older people and others with compromised immune systems.

Can I get the flu from the flu shot?

A lot of people seem to think so, but the short answer is, almost certainly not. Most versions of the flu vaccine contain inactivated viruses. A dead virus can’t reproduce, but your immune system can still react to the protein and form antibodies that will react if you later come into contact with a live virus. This year’s nasal spray flu vaccines have live attenuated viruses, which are weakened versions of the virus. If you have a healthy immune system, you shouldn’t get sick from it, but avoid the flu mist if you have a severely compromised immune system (or come into close contact with someone who does, since you might shed the virus for a short period of time). There’s also a new recombinant vaccine that has the protein your immune system reacts to but no actual flu virus bodies, so you definitely can’t get infected from it.

Can I get the flu after I get the flu shot?

Yes, that’s a different question. And the answer is — maybe. There are a few different ways you can still get sick after being vaccinated, but don’t blame the shot!

  • You were already infected when you were vaccinated, or were exposed shortly after getting vaccinated. Flu symptoms don’t show up immediately after you get infected; in most cases, the incubation period is 1-4 days. If you got the shot and then got sick the next day, you probably just had really lousy timing. (If you waited until someone sick coughed all over you and then you got vaccinated to try to keep from catching it from them, it doesn’t work that way. Sorry!) Also, your body takes about two weeks to build up full immunity after getting vaccinated, so you can get sick if you’re exposed during that period. The earlier in the flu season you get vaccinated, the lower your chances are of either of these scenarios happening, so don’t procrastinate!
  • You caught a different strain of the flu. Influenza mutates quickly, and vaccines can’t cover every single strain in the world. Researchers make their best guess about which strains will be most common and the vaccines are made to cover those 3-4 versions of the flu virus. Unfortunately, sometimes a new strain pops up and becomes widespread after the vaccines are in production, or you might have caught one of the rarer versions.
  • You’re sick, but not with the flu. People frequently assume any respiratory infection during flu season is the flu, but you might just have a nasty cold or a different viral or bacterial infection. Also, the “stomach flu” isn’t the flu at all. If you’re experiencing vomiting and diarrhea, you probably don’t have an influenza infection.
  • You’re having mild side effects. If you’re a little achy or have a mild fever the day after you get vaccinated, you aren’t sick. You’re just feeling the effects of your immune system kicking into gear.

What about those side effects?

Minor side effects are fairly common, but are still less serious than actually getting the flu. If you get the shot, the most common reaction is swelling, redness, and/or soreness at the injection site. (Which makes sense, because you are, in fact, poking a needle into your muscle and injecting fluid. That’s gonna hurt a little bit no matter what’s in the syringe.) Other possible problems include “fainting (mainly adolescents), headache, muscle aches, fever, and nausea.” The nasal spray can also cause respiratory symptoms such as a runny nose, sore throat, or cough.

Serious side effects from the flu vaccine are extremely rare. If you’re severely allergic to a component of the shot, a reaction will usually set in within a few minutes to a few hours after vaccination. These can be life-threatening, so contact your doctor immediately if you experience “difficulty breathing, hoarseness or wheezing, swelling around the eyes or lips, hives, paleness, weakness, a fast heart beat or dizziness” or a sudden high fever. There’s also a possible link between the flu vaccine and Guillan-Barré Syndrome, which is most common in people over age 50. However, the CDC estimates that only 3,000-6,000 people in the U.S. develop GBS each year, whether or not they were vaccinated, so the odds are vanishingly small of it being a cause for concern.

What about egg allergies?

If you have a severe egg allergy, you can still receive the recombinant vaccine (if you’re 18 or older), since it isn’t cultured in eggs. You may also be able to get the inactivated vaccine if your doctor is prepared to treat any possible reaction; be sure to discuss it with them to see what they think the best course of action is. If you’ve only ever had a mild case of hives from eating eggs, you may also be ok, but again, alert your doctor ahead of time. (My daughter’s former pediatrician, who went to med school in Russia, pretty much scoffed when we asked if it was a cause for concern, but others may be more cautious.)

However, you might not actually be allergic to eggs, even if you think you are. A true allergy can cause itching, a rash or hives, trouble breathing or swallowing, chest pain, or a drop in blood pressure, and these symptoms will happen every time you eat anything containing eggs. If your usual symptoms after consuming eggs are purely gastrointestinal and you only react sporadically or only after eating a large amount, you likely have an egg intolerance. Since that doesn’t involve your immune system, it’s not an excuse to skip vaccination.

What about mercury in vaccines?

Yes, multi-dose vials of the flu vaccine do contain thimerosal, which is a mercury-based preservative. However, there’s still absolutely no cause for concern. Thimerosal can metabolize into ethylmercury,* which doesn’t bioaccumulate (in other words, it’s cleared out of your body fairly quickly). Methylmercury is the more dangerous form that can build up in the food chain and cause mercury poisoning because it doesn’t leave the body easily. There isn’t enough mercury in a single shot to cause any adverse health effects, and since it leaves the body quickly, it doesn’t add up over time if you get the flu shot each year. There’s also absolutely no link between thimerosal and autism. If you’re still concerned for whatever reason, you can still use the nasal flu vaccine or a single-dose injection if your doctor has them, since those versions don’t use preservatives.

*I know the chemical prefixes don’t mean much to most people, but think of it like the difference between ethyl alcohol (ethanol) and methyl alcohol (methanol). Ethanol is in all alcoholic beverages and is basically safe to consume in moderation; ingesting even a tiny amount of methanol can quickly cause blindness or death since it metabolizes into formaldehyde and then formic acid. A tiny change in the chemical formula can make a huge difference.

But I got the shot last year!

Sorry, you still need a new one. As mentioned above, the flu mutates quickly and different strains are common each year. Even if you’re immune to previous years’ flu strains, there’s probably a new strain that you aren’t covered for. Of course you don’t have to get a shot each year, but getting the flu sucks much worse than just getting the vaccine.

If money’s a concern, keep in mind that the Affordable Care Act requires insurance companies to cover flu shots (and many other vaccines) with no copay or hit to your deductible so long as you get them in-network. If you don’t have insurance, look for local programs that provide free or low-cost vaccines. After all, treating the flu is much more expensive than the vaccine, especially when you take into account the time you might have to miss work or school to recover or care for sick family members.

But I still don’t know what to do!

Talk to your doctor. If you’re still concerned about the vaccine in general, or you have a specific health concern such as pregnancy or a chronic condition, they’ll help you figure out the safest course of action.

By [E] Hillary

Hillary is a giant nerd and former Mathlete. She once read large swaths of "Why Evolution is True" and a geology book aloud to her infant daughter, in the hopes of a) instilling a love of science in her from a very young age and b) boring her to sleep. After escaping the wilds of Waco, Texas and spending the next decade in NYC, she currently lives in upstate New York, where she misses being able to get decent pizza and Chinese takeout delivered to her house. She lost on Jeopardy.

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