On the day that Shelly Kendeffy got her second dose of the Moderna vaccine, the side effects hit her hard. She had a sore arm, body aches, and by evening it felt like the flu.
“My teeth were chattering, but I was sweating,” said Shelly, a medical technician in State College, Pa.
By the next day she had recovered enough to go to work. She asked her colleagues—eight men and seven women–how they felt. All seven of the women had symptoms at least as severe as Shelly did.
The eight men had far different stories. Four had no symptoms at all. The other four had mild fatigue or achiness.
It turns out that the gender side effect gap that Shelly noticed is playing out across the country. A new study by the Centers for Disease Control and Prevention found that women suffer coronavirus vaccine side effects far more often and more severely than men.
The COVID Vaccine Side Effect Gender Gap
Researchers analyzed safety data from the first 13.7 million vaccine doses given to Americans. More than 79% of the reported side effects came from women, even though only 61% of the vaccines were given to women.
The most serious side effect of the vaccines has been rare instances of severe allergic reactions. Nearly all of them have occurred in women. The CDC study found that all 19 people who experienced anaphylactic reactions to the Moderna vaccine were female. Women made up 44 of the 47 people who had severe allergic reactions to the Pfizer shots.
The sex difference in side effects is actually not unusual. It happens with other vaccines as well.
A study in the journal Vaccine found that four times as many women as men suffer flu vaccine side effects. Another study found that women accounted for 80% of all anaphylactic reactions to all vaccines given in the U.S. between 1990 and 2016. 
The news isn’t all bad for women…
Studies have found that vaccines may provide more protection for them than men. They produce up to twice as many antibodies in response to vaccines for flu, MMR, and hepatitis.
The same likely holds true for coronavirus vaccines, said Dr. Sabra Klein of the Johns Hopkins Bloomberg School of Public Health, although there have not yet been studies to confirm this.
“This sex difference is completely consistent with past reports of other vaccines,” she said.
The important thing is that women know what to expect, Dr. Klein said. They should be prepared to feel under the weather for a day or two after getting vaccinated.
“That is normal, and likely reflective of their immune system working,” she said.
No Side Effects? You’re Still Getting Protection
So does this mean that if you don’t suffer side effects that the vaccine is not working?
Lab tests show that people who don’t suffer side effects do build antibodies against COVID-19.
But we actually don’t know if side effect sufferers get a higher level of protection. That’s because there have been no clinical studies comparing the effectiveness of the vaccines in people who had side effects with those who didn’t.
Dr. Gregory Poland is an infectious disease specialist and head of Mayo Clinic’s Vaccine Research Group. He said that if people don’t get side effects, or if they are mild, it does not mean they aren’t developing an immune response.
“Each of our bodies releases different amounts of chemicals, or immune signals” after getting the vaccine, Dr. Poland said. “One body might release more than what’s needed, causing more (side effects).
“Someone else’s body might release exactly the right amount. It’s what is called the ‘Goldilocks phenomenon.’ Not too much, not too little, but just right.”
The bottom line?
You’re getting some level of immunity whether you’re hit with side effects or not.
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