By Jason Gale and Robert Langreth | Bloomberg

Moderna Inc.’s Covid vaccine generated more than double the antibodies of a similar shot made by Pfizer Inc. and BioNTech SE in research that compared immune responses evoked by the two inoculations.

The study is one of the first to compare levels of antibodies produced by the two vaccines, which are thought to be one of the important components of the immune response. It didn’t examine whether the antibody differences led to a difference in efficacy over time between the two shots, which both were more than 90% effective in final-stage clinical trials.

The research looked at antibody levels against the coronavirus spike protein in about 1,600 workers at a major Belgium hospital system whose blood samples were analyzed 6 to 10 weeks after vaccination. The participants hadn’t been infected with the coronavirus before getting vaccinated. Levels among those who got two doses of the Moderna vaccine averaged 2,881 units per milliliter, compared with 1,108 units per milliliter among those who received two Pfizer doses.

The results, published Monday in a letter to the Journal of the American Medical Association, suggested the differences might be explained by the higher amount of active ingredient in the Moderna vaccine — 100 micrograms, versus 30 micrograms in Pfizer-BioNTech — or the slightly longer interval between doses of the Moderna vaccine — four weeks, versus three weeks for Pfizer-BioNTech.

Outside researchers said it was premature to conclude that the difference in antibody levels was medically important.

“I would urge caution in making the conclusion that because Moderna demonstrated a slightly higher peak on average that its efficacy will be slower to wane,” said David Benkeser, a biostatistician at Emory University, in an email. “Such a conclusion requires a host of assumptions that have not yet been evaluated.”

Both vaccines produce high levels of antibodies, he noted, and other studies have shown even relatively low levels of antibodies are protective.

Still, it’s possible that higher initial antibody levels might correlate with longer duration of protection against mild breakthrough infections, said Deborah Steensels, a microbiologist at Ziekenhuis Oost-Limburg, a large hospital in Belgium, who was lead author on the study.  Also, if higher antibody levels are confirmed to be important, then the Moderna vaccine might be better for immunocompromised people who don’t respond well to vaccines, she said.

Pfizer said in a statement that its vaccine “continues to be highly efficacious” in preventing Covid-19, including against severe cases and hospitalization. A continuing analysis of its final-stage study has shown a decline of efficacy against symptomatic infection over time, the drugmaker said, but initial trial data also show that a third dose of the existing vaccine at least six months after the first two significantly raises neutralizing antibody levels.

Moderna’s vaccine was associated with a two-fold risk reduction against breakthrough SARS-CoV-2 infections compared to Pfizer’s in a review of people in the Mayo Clinic Health System in the U.S. from January to July. The results were reported in a separate study released ahead of publication and peer review on Aug. 9.

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