Moderna COVID-19 vaccine's safety slightly bests Pfizer's

Elderly man getting COVID vaccine
Elderly man getting COVID vaccine

US Air Force, Sarah M. McClanahan / Flickr cc

An observational study today in JAMA Internal Medicine reports a slightly better safety profile for the Moderna mRNA COVID-19 vaccine than for the Pfizer/BioNTech version in US veterans, but both vaccines had very good safety profiles.

A team led by Harvard University researchers reviewed the electronic health records of 433,672 US Department of Veterans Affairs patients across the country who received their first dose of the Pfizer or Moderna COVID-19 vaccine from Jan 4 to Sep 20, 2021, with a second dose scheduled for 21 to 28 days later, depending on the vaccine.

Median patient age was 69 years, 93% were men, 20% were Black, and 8% were Hispanic. Median follow-up was 223 days.

Very low risks with both vaccines

Of Pfizer vaccine recipients with at least 21 days of follow-up, 92% received a second dose before day 24, and 96% received it before day 28. Among Moderna recipients with at least 28 days of follow-up, 92% received it before day 31, and 97% did so before day 35.

Relative to the Moderna group, Pfizer recipients had an excess per 10,000 people of 10.9 ischemic strokes (95% confidence interval [CI], 1.9 to 17.4), 14.8 heart attacks (95% CI, 7.9 to 21.8), 11.3 other abnormal blood clotting (95% CI, 3.4 to 17.7), and 17.1 cases of kidney injury (95% CI, 8.8 to 30.2).

The corresponding risk ratios for Pfizer versus Moderna were 1.17 (95% CI, 1.03 to 1.28) for ischemic stroke, 1.32 (95% CI, 1.16 to 1.49) for heart attack, 1.20 (95% CI, 1.05 to 1.32) for other blood-clotting events, and 1.16 (95% CI, 1.08 to 1.29) for kidney injury.

In analyses that didn't exclude COVID-19–related adverse events, estimated risk differences were only marginally higher except for that for pneumonia, which rose from 5.9 (95% CI, −2.9 to 12.7) to 20.9 (95% CI, 11.4 to 29.5) events per 10,000 participants.

In the 14 days after the first vaccine dose, the estimated risk of adverse events were low and comparable between the groups. In the first 42 days, the estimated risks were also low, but relative to Moderna recipients, Pfizer recipients had excess abnormal blood-clotting events (1.8; 95% CI, 0.2 to 4.2), heart rhythm abnormalities (7.5; 95% CI, 1.9 to 11.5), and kidney injuries (3.8; 95% CI, 2.1 to 8.0) per 10,000 people.

Black veterans at much higher risk for blood clots

Estimated excess events were mostly comparable among age-groups and races, but older and White participants were at higher risk for ischemic stroke (15.5 [95% CI, 3.2 to 23.2] and 10.5 [95% CI, 2.4 to 20.40, respectively)

Also, older people had higher odds of kidney injury (24.4; 95% CI, 11.7 to 42.6), and Black participants were at much higher odds of abnormal blood clots (38.7; 95% CI, 15.8 to 49.0).

The researchers said the results offer a real-world, head-to-head comparison with a longer follow-up and a larger, more diverse population than randomized clinical trials of the vaccines.

"Although the primary analysis was designed to detect safety events unrelated to SARS-CoV-2 infection, the possibility that these differences may partially be explained by a lower effectiveness of the BNT162b2 [Pfizer] vaccine in preventing the sequelae of SARS-CoV-2 infection compared with the mRNA-1273 [Moderna] vaccine could not be ruled out," the authors wrote. "These findings may help inform decision-making in future vaccination campaigns."

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