September 01, 2021
The study compared the levels of antibodies produced against the coronavirus spike protein. However, the research didn't measure the levels of neutralizing antibodies or whether the differences correlated with vaccine efficacy over time.
"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," David Benkeser, a biostatistician at Emory University, told Bloomberg News.
"Such a conclusion requires a host of assumptions that have not yet been evaluated," he said.
The study evaluated antibody levels in 1,647 workers at a major Belgium hospital system. The researchers analyzed blood samples about 6-10 weeks after vaccination.
Among those who had not been previously infected, the Moderna recipients average 2,881 units per milliliter, as compared with Pfizer recipients who averaged 1,108 units per milliliter.
Those who previously contracted COVID-19 had higher antibody levels, with Moderna recipients averaging 3,836 units per milliliter and Pfizer recipients averaging 1,444 units per milliliter.
Across all ages, those who weren't previously infected with COVID-19 and were vaccinated with the Moderna vaccine had higher antibodies than those vaccinated with the Pfizer vaccine, although the highest antibodies were seen in ages 35 and younger.
The differences could be explained by the higher amount of active ingredient in the Moderna vaccine, the researchers wrote. The Moderna vaccine has 100 micrograms of active ingredient, as compared with 30 micrograms in the Pfizer vaccine.
The slightly longer interval between doses could lead to differences as well, the researchers wrote. The Moderna shots are taken four weeks apart, while the Pfizer shots are taken three weeks apart.
Now the study team wants to determine whether the different antibody levels correlate with vaccine efficacy and longer protection, and if so, whether the Moderna vaccine may be better for immunocompromised people who don't respond as well to vaccines, Bloomberg reported.