Pfizer COVID vaccine shows 78% efficacy in pregnancy

Pregnant woman getting vaccinated
Pregnant woman getting vaccinated

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Two doses of the Pfizer/BioNTech COVID-19 vaccine were safe and 78% effective in preventing infection in pregnant women in a real-world study in Israel.

Led by researchers at Maccabi Healthcare Services in Tel Aviv, the retrospective, observational study was published yesterday in JAMA. It involved analyzing data on 15,060 women in a pregnancy registry of a large, state-mandated healthcare system who were vaccinated with a first dose from Dec 19, 2020, to Feb 28, 2021.

The women were matched in a 1:1 ratio with unvaccinated pregnant women by age, gestational age, residential area, population subgroup, number of times they had given birth, and flu vaccination status (a proxy for health-seeking behavior) and followed until Apr 11, 2021.

Risk dropped in vaccinees over time

Among the 7,530 vaccinated women, 118 (1.6%) were infected with SARS-CoV-2 at least 28 days after their first dose of the BNT162b2 vaccine, compared with 202 in the 7,530 (2.7%) unvaccinated women. Cumulative incidence was 1.85% among vaccinees and 3.90% among unvaccinated women, a difference of 2.05 percentage points. COVID-19–related hospitalization rates were 0.2% in vaccinated women and 0.3% in the unvaccinated.

"In this large population-based cohort of pregnant women, BNT162b2 vaccination compared with no vaccination was associated with a significantly lower risk of SARS-CoV-2 infection, although the absolute risk difference was small," the study authors wrote.

Eighty-eight of 105 infected vaccinated women (83.8%) had coronavirus symptoms, versus 149 of 179 (83.2%) in the unvaccinated women, a nonsignificant difference.

At 28 days, 4,788 of 7,530 women (63.6%) remained at follow-up in each group. The absolute cumulative number of events was 109 in vaccinated women and 158 in the unvaccinated, and the difference in cumulative incidence was 0.80%. Cumulative incidence was 1.55% in the vaccinated group and 2.34% in the unvaccinated.

Over 28 to 70 days of follow-up, 10 vaccinated and 46 unvaccinated women tested positive for coronavirus (hazard ratio [HR], 0.33% vs 1.64%, respectively), for an absolute difference of 1.31% and an adjusted HR (aHR) of 0.22.

Risk reduction rose over time after vaccination, as indicated by a time-varying covariate. In the 10 days after vaccination, there was no significant difference between aHRs (0.96) in the two groups of women. But among vaccinees, there was a statistically significant hazard reduction from 11 to 27 days after vaccination (aHR, 0.46).

During follow-up, 18.4% of the vaccinated group and 18.9% of the unvaccinated group reached the end of pregnancy. There were no significant differences between the groups in terms of rates of preeclampsia (maternal high blood pressure and organ damage), smaller-than-expected babies for gestational age, birth weight, abortion, stillbirth, maternal death, or pulmonary embolism (blockage of a pulmonary artery).

No severe adverse events

While 68 patients reported vaccine-related adverse events, none were severe. The most common side effects were headache (10 [0.1%]), weakness (8 [0.1%]), unspecified pain (6 [less than 0.1%]), and stomach ache (5 [less than 0.1%]). All symptoms resolved within 1 day.

Among vaccinated women, 46% were in their second or third trimester, compared with 33% in the unvaccinated group. Mean age was 31.1 years, and the median follow-up for SARS-CoV-2 infection was 37 days.

The authors noted that changes in hormone levels and immune function in pregnancy may raise the risk of viral infection. And while COVID-19 infection in pregnancy is usually asymptomatic or mild, it can result in admission to an intensive care unit and mechanical ventilation, especially in the third trimester. Symptomatic infections have also been tied to higher odds of preterm delivery and fetal distress during birth.

The study was particularly important, the researchers said, because while the Pfizer vaccine showed 85% efficacy in preventing infection 7 days or more after the second dose in a phase 3 clinical trial, pregnant women were not included.

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