PASCO COUNTY, Fla. — Fatigue, muscle aches, and headache are usually signs it might be time to get some rest and drink plenty of fluids, but experts say in the case of Moderna's COVID-19 vaccine, they're not necessarily a bad thing.

What You Need To Know

  • Doctors say public should be prepared for possible vaccine side effects

  • Moderna says severe effects reported during its clinical trial included fatigue, muscle aches, and joint pain

  • Hospital epidemiologist with UF says people should plan timing of doses for days they're not busy in case they do experience side effects

"The side effects that we experience from the flu or many of these infections are not from the virus itself — they're from the immune system getting activated," said Dr. Kartik Cherabuddi, an associate professor of infectious diseases and hospital epidemiologist with the University of Florida.

Triggering that kind of response is exactly what the vaccine is meant to do.

According to Moderna, most side effects reported during its clinical trials were mild to moderate, but some were classified as "Grade 3", or severe.

After the first dose, 2.7% of clinical trial participants experienced this level of injection site pain. After the second dose, 9.7% experienced fatigue, 8.9% reported muscle aches, 5.2% had joint pain, 4.5% had headache, 4.1% reported pain, and 2% saw redness at the injection site. 

"So, flu-like side effects and injection-related side effects are what you should expect," said Cherabuddi. "I would make it clear that they're a little more than the flu vaccine."

According to Pfizer, no serious safety concerns have been observed with its vaccine candidate to date.

CNN reported last week that doctors told members of the Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices that it will be important to make sure people getting any eventual vaccine know what to expect. Cherabuddi said it's a good idea to plan the timing of doses as much as possible.

"Time will tell, but you know that you shouldn't have your most active day be the hours after the vaccine or the next day," he said.

Cherabuddi said there is a concern some people might not get an approved vaccine because of possible side effects.

"In some ways, I would look at it as a check mark and say, 'Yes, I got the vaccine, my immune system is working and doing its job, I just have to let it and prepare for some of the side effects,'" he said. "I would look at it, if you can, as a positive thing. I don't want to downplay these effects and what they can do to a person's ability to work, but it's your best indicator that the vaccine is working."

Moderna said it's still gathering safety data and the study is being monitored by an independent, National Institutes of Health-appointed Data Safety Monitoring Board.