Pfizer’s RSVpreF Vaccine Effective in Preventing Severe Infections in Infants During Pregnancy

A bivalent RSV prefusion F protein–based (RSVpreF) vaccine has shown effectiveness against medically attended severe RSV-associated lower respiratory tract infection in infants when administered during pregnancy, according to a recent study published in The New England Journal of Medicine. The study's primary endpoint, medically attended severe RSV-associated lower respiratory tract illness, was met with a reported efficacy of 82%, and no safety concerns were identified. RSV is a common respiratory viral infection that usually carries mild symptoms but can become severe in young children and older adults. In fact, RSV is the most common cause of hospitalization in children under five years of age, and even more so in those younger than six months.[0]

The phase 3, double-blind trial was conducted in 18 countries and randomized 7,392 women in their third trimester of pregnancy to receive either a single intramuscular injection of 120 μgbivalent RSVpreF vaccine or placebo.[1] The trial participants were healthy women aged 49 years or younger, at 24-36 weeks’ gestation, with an uncomplicated single pregnancy and no known increased risk of pregnancy complications.[1] The two primary efficacy endpoints were medically attended severe RSV-associated lower respiratory tract illness and medically attended RSV-associated lower respiratory tract illness in infants within 90, 120, 150, and 180 days after birth.

While RSV may not be severe for many, it can lead to hospitalization or even death for vulnerable groups such as infants and the elderly. In a single year, there are about 34 million episodes of RSV-associated lower respiratory tract infections in children under the age of 5, and about 10% need to go to the hospital for treatment.[2] RSV is also the No. 1 reason infants in the US have to go to the hospital.

The study authors noted that there were no safety signals detected in maternal participants or in infants and toddlers up to 24 months of age.[3] The occurrence of unfavorable incidents within one month of injection or one month after birth had comparable rates in the vaccine and placebo groups. In the vaccine group, 13.8% of women and 37.1% of infants reported adverse events, while the placebo group reported 13.1% and 34.5%, respectively. Serious maternal adverse events through 6 months were similar between groups, with the most frequent being preeclampsia (1.8% and 1.4% of vaccine and placebo participants, respectively) and fetal distress syndrome (1.8% vs 1.6%). There were no significant negative occurrences in infants that were associated with the RSVpreF vaccine.

Pfizer's RSVpreF vaccine was given to mothers in the second half of their pregnancy and was effective in preventing severe infections in infants. The vaccine can also protect older people with poor health who get flu-like symptoms and respiratory infections from RSV.[4] The vaccine manufactured by Pfizer has been proven to be successful in preventing severe lower respiratory tract illness associated with RSV in infants even when administered during pregnancy. Additionally, there were no safety issues detected during the study. A key advisory committee for the U.S. Food and Drug Administration recommended approval of the Pfizer vaccine for seniors and a competing RSV vaccine developed by GlaxoSmithKline.[2]

The RSV vaccine could be available by this fall, and some contenders are showing promising signs in trials right now. The world's first-ever RSV vaccine for adults could soon be available. The European Union will likely register this vaccine next year.[5]

0. “Respiratory virus plagues South Africa, but new vaccine for pregnant moms saves babies” Medical Xpress, 6 Apr. 2023,

1. “RSV Vax in Pregnancy Cuts Severe Illness in Infants” Medpage Today, 6 Apr. 2023,

2. “Late-stage RSV vaccine trials show ‘exciting promise,' some scientists say” KVOA Tucson News, 5 Apr. 2023,

3. “Bivalent Prefusion F Vaccine in Pregnancy to Prevent RSV Illness in Infants | NEJM”, 5 Apr. 2023,

4. “New RSV vaccine could prevent severe illness in seniors and infants” CBS News, 6 Apr. 2023,

5. “Breakthrough against RS virus: Two safe, effective vaccines developed” NL Times, 6 Apr. 2023,

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