By Dr. Catherine Schuster-Bruce 

Pfizer's top scientist said it chose a relatively low COVID-19 vaccine dose to minimize side effects. Pfizer's vaccine has 30 micrograms of mRNA, the active ingredient. Moderna's has 100 micrograms. Both vaccines produce similar side effects, according to the CDC.

Pfizer's top scientist defended the dose of the active ingredient in the company's COVID-19 vaccine, which is lower than in Moderna's vaccine.

Pfizer's COVID-19 vaccine has 30 micrograms of mRNA, while Moderna's has 100 micrograms. Scientists have speculated that this could be a reason Pfizer's shot produced a lower antibody response than Moderna's in recent studies.

Philip Dormitzer, Pfizer's chief scientific officer said on Wednesday that Pfizer and its codeveloper BioNTech "used the minimum dose level" to get an immune response that was stronger than catching COVID-19.

Dormitzer added that a higher dose might have risked more side effects.

"If you look at what's going on with all the COVID-19 vaccines out there, the derailer has often been adverse events that have cropped up," he said.

Pfizer's and Moderna's COVID-19 vaccines produce similar side effects, according to the Centers for Disease Control and Prevention. They can include arm pain, soreness, and redness; muscle aches; fatigue; and fever.

The CDC has said that a type of heart inflammation called myocarditis is an "extremely rare" side effect of both vaccines that resolves quickly.

There have been reports of so-called Moderna arm, an angry red rash that appears after getting Moderna's shot and goes away on its own.

In the US, providers have given more than 214 million doses of Pfizer's shot and 147 million doses of Moderna's, according to the CDC.

Moderna's shot boosted antibodies higher than Pfizer's in some studies

A study of 1,600 Belgian health workers published as a research letter in the Journal of the American Medical Association on August 30 found that Moderna's COVID-19 vaccine produced twice as many antibodies as Pfizer's at six to 10 weeks after vaccination.

The authors said that the higher mRNA levels in Moderna's vaccine and a longer interval between doses "might explain this difference."

A study from the University of Virginia published as a research letter on September 2 found no difference in antibody response among age groups with Moderna's vaccine but a lower antibody response in people 50 and older with Pfizer's. The researchers said that the differences could relate to the amount of mRNA in the vaccines.

The antibody response is just one aspect of the immune system, and the antibody level needed to protect against COVID-19 has not been established.

Vaccine protection also depends on whether the antibody response changes over time. For example, a recent study from Oxford University found that at four months, Pfizer's and AstraZeneca's vaccines produced similar levels of antibodies. The antibody levels from Pfizer's shot had waned, while the levels from AstraZeneca's remained the same, the researchers said.

Real-world data from Canada earlier this year found that after one dose, Pfizer's vaccine was 56% effective against symptomatic COVID-19 caused by the highly infectious Delta variant, while Moderna's was 72% effective.

Other factors could have influenced the results, including that Pfizer's vaccine had typically been given to older people, who tend to produce weaker immune responses. That study hasn't been scrutinized by experts in a peer review.