With the end of summer comes the beginning of a new season: flu season. It's a time to load up on tissues and head to the nearest pharmacy or doctor's office for your annual flu shot. And this year, public health officials are putting even more emphasis on getting vaccinated to prevent a "twindemic" of COVID-19 and influenza. Here’s what you need to know about the flu shot.

1. The flu is a nasty viral disease.

“Flu” is short for influenza, which is a disease caused by the influenza virus. The virus affects a person’s lungs, nose, and throat, so symptoms will be concentrated in those areas. The most common symptoms are fever, cough, sore throat, runny or stuffy nose, muscle aches, and fatigue. Not everyone will have all of these symptoms. Flu season usually starts in October, peaks in January or February, and ends by May.

2. "Stomach flu" is not real.

There's no such thing as "stomach flu." The nausea, vomiting, and diarrhea that are commonly called “stomach flu” can be caused by a virus, bacteria, or parasite, but not the influenza virus. Occasionally, the flu virus can cause nausea or vomiting, but this is far more common in children than adults. This may go without saying, but if you’ve been vomiting or have had diarrhea for a few days, it’s time to see a doctor.

3. There are many different strains of the flu.

The flu virus comes in numerous strains, or types. The strain called H1N1 started in pigs, then spread to humans, and is now a common type of seasonal flu. The bird flu, also known as H5N1 or H7N9, has made a lot of birds sick, but rarely spreads to humans unless they’ve handled infected birds.

4. A flu shot contains a tiny particle of the dead virus.

Each shot contains a teeny tiny bit of dead flu virus. The virus is grown in fertilized chicken eggs, then extracted and deactivated with microscopic amounts of formaldehyde. A chemical called octylphenol ethoxylate pulls out even smaller pieces of virus, which helps reduce the chances of side effects. Gelatin holds the virus together and keeps it stable during shipping, and a preservative called thimerosol keeps the vaccine from going bad on the shelf. There’s no reason to be concerned about any of these chemicals; they’re present in such small quantities that your body will barely register them. If you have a life-threatening allergy to gelatin or eggs, talk to your doctor before you get your shot. He or she may recommend a different version. (Also, see #9.)

5. You should get a flu shot even if you think you never get the flu.

Past performance is not indicative of future results, my friend. Just because you’ve never had it before doesn’t mean you’re invincible. In addition, even if you never have symptoms in your life, you could be carrying the virus around, exposing everyone else to it. And not everybody’s immune system is as robust and macho as yours. Think about babies, and people with compromised immune systems, and pregnant people, and the elderly. Do you really want to be the one who gets them sick?

6. Yes, you need to get a flu shot every year.

There are many, many types of flu. Each year, researchers and public health officials determine which strains seem like they’re going to be a threat, and formulate a vaccine that protects against those strains. To stay protected against the latest flu risks, you must keep your shots up to date.

7. This year's flu shots will protect against three or four strains.

According to the Centers for Disease Control and Prevention, three or four kinds of flu viruses commonly circulate among people today: influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses. The 2020-2021 flu shot has been updated to protect against three virus strains: A/Guangdong-Maonan/SWL1536/2019 (H1N1) pdm09-like virus, A/Hong Kong/2671/2019 (H3N2)-like virus, and B/Washington/02/2019 (B/Victoria lineage)-like virus.

Quadrivalent flu shots, which are designed to protect against four types of flu, will protect against an additional B virus called B/Phuket/3073/2013-like (Yamagata lineage) virus.

8. The flu shot can't give you the flu.

Your flu shot is either made with dead (deactivated) flu virus or, in the case of the recombinant flu vaccine, with no actual virus at all. You may have some side effects after getting your shot, but those are usually limited to pain or swelling around the site of the injection. In rare cases, you may have a low-grade fever or mild muscle aches, but these are side effects, and not the flu.

9. You can get the flu shot if you're allergic to eggs.

For a while, doctors were cautioning people with egg allergies to stay away from the flu vaccine, but this seems to have been unnecessary. The American Academy of Allergy, Asthma, and Immunology recently stated that “no special precautions are required for the administration of influenza vaccine to egg-allergic patients, no matter how severe the egg allergy.” If you’re really concerned about an allergic reaction, talk to your doctor. He or she may be able to get you an egg-free flu shot.

10. If you get the flu, antibiotics won't help.

The flu is caused by a virus, not bacteria; antibiotics respond only to bacteria. Antibiotics won’t do anything to fight the flu virus, but they will mess up your body’s bacterial ecosystem and hasten the growth of antibiotic-resistant superbugs.