• Pfizer and Moderna COVID-19 vaccines could produce more side effects
  • Both vaccines use mRNA technology 
  • Doctors say Americans shouldn't skip the second dose to avoid side effects

Americans who received shots of the COVID-19 vaccine from Pfizer or Moderna are more likely to experience side effects, a vaccinologist said Thursday. 

The Pfizer/BioNTech and Moderna vaccines both use messenger RNA (mRNA) technology to provide up to 95% protection against symptomatic COVID-19 and 100% protection against severe cases of the illness. However, both may produce more side effects than other vaccines. 

“The mRNA vaccines -- the Pfizer and the Moderna -- they are what's called more reactogenic, meaning there are more side effects,” Dr. Peter Hotez, a vaccinologist and dean of the National School of Tropical Medicine at Baylor College of Medicine, told CNN.

“They're not serious side effects, but they can be unpleasant, and they can sometimes last a day or two. Usually they don't. Usually they last a few hours,” he added. 

Doctors and health experts warn recipients not to panic as side effects from the vaccine mean it's working. 

“The kinds of things we're seeing are arm soreness, body aches, sometimes fatigue, sometimes even low-grade fever,” Hotez said. “And we know why this happens -- because the vaccine is very potent in inducing an immune response. That's one of the reasons why we're getting such high levels of protection.”

It is not certain how many people get side effects, but in vaccine trials, 10% to 15% of participants developed noticeable side effects, former Operation Warp Speed Chief Scientific Adviser Moncef Slaoui said last year.

The CDC said the side effects may be more intense after receiving the second shot. 

Among Pfizer clinical trial participants, 3.8% experienced fatigue and 2% got a headache after receiving both shots. Moderna says 9.7% of its participants developed fatigue and 4.5% got a headache. 

People who received a shot of Johnson & Johnson’s coronavirus vaccine reported experiencing pain at the injection site and a flu-like fever. 

The CDC said the three authorized vaccines share several common side effects, including pain, redness, swelling, muscle pain, chills and nausea. Officials for the health agency warned against skipping the second shot to avoid side effects. 

"You should get the second shot even if you have side effects after the first shot, unless a vaccination provider or your doctor tells you not to get it,” the CDC said. 

Some ten percent of Americans have so far received at least one Covid vaccine dose, with Moderna accounting for just under half the number Moderna logo. Photo: AFP / JOEL SAGET