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Pfizer Reports 87% Efficacy for Its RSV Vaccine in Adults

(Reuters Health) – Pfizer’s respiratory syncytial virus (RSV) vaccine RSVpreF has an efficacy of 86.7% in healthy adults ages 18 to 50, according to results from a small phase 2a study published in Thursday’s New England Journal of Medicine.

The test was done on 70 adults who were pre-screened to make sure they were vulnerable to RSV infection. They received a single injection of the vaccine or placebo, with a challenge from the RSV A Memphis 37b strain 28 days later. The patients were then observed for 12 days in a quarantine unit.

Aside from injection site pain, the researchers report finding no significant side effects from the shot. The safety follow-up lasted 6 months.

Pfizer, which ran and paid for the test, is one of several companies – including AstraZeneca, GSK, Johnson & Johnson, Moderna and Sanofi – working on a vaccine or therapy for the virus, which is responsible for about 177,000 hospitalizations and 14,000 deaths each year in the United States among people aged 65 and older.

Among all adults, RSV makes 3% to 10% fall ill each winter. It is also a leading cause of pneumonia among toddlers. Among children under 5 it causes about 58,000 hospitalizations per year.

Whether the vaccine will work as well in people who are not young and healthy is not known.

A phase 3 field trial, known as RENOIR, began in August. That investigative team plans to enroll 45,000 volunteers ages 60 and older at 265 centers in seven countries. The estimated completion date is June of 2024. A phase 3 study in which the vaccine is being given to pregnant women in hopes that it protects their babies from RSV is ongoing.

Although the virus was first identified in 1956, vaccine development has been difficult because of the virus’ complex molecular structure.

The 86.7% efficacy estimate is based on a lack of symptoms along with confirmation by the absence of any detectable viral RNA for at least two consecutive days.

While none of the vaccine recipients had symptoms, 13 of 31 placebo recipients who went on to receive the challenge infection felt ill. Among placebo patients, their viral load was positive from days 3 through 12, peaking at day 6.

“In the RSVpreF vaccine group, the viral load was low to undetectable,” writes the research team, led by Dr. Beate Schmoele-Thoma of Pfizer Pharma, Berlin.

Viral shedding lasted for a median of 18.0 hours in the vaccine group versus 131.6 hours in the placebo population.

The side effect profile is unclear. The team said at least one unsolicited adverse event was reported during the 28 days after the shot in 34% of vaccine patients and 29% of placebo patients. But just one solicited “systemic event” – fatigue within 7 days of the intervention – appeared in 40% of vaccine recipients and 30% of placebo patients.

The study’s senior author and a Pfizer spokesperson declined to comment in response to a Reuters Health request for clarification.

One patient in the vaccine group developed enlarged submandibular lymph nodes on day 26. The swelling had gone away by day 53, the researchers reported.

In an accompanying editorial, Dr. Marie Griffin of the Vanderbilt University School of Medicine in Nashville, Tennessee, noted that “the challenge occurred shortly after vaccination, so the duration of protection is unknown.

“Whether the vaccine will be able to prevent lower respiratory tract disease in persons at higher risk for RSV-associated serious illness has yet to be determined,” she added. “In addition, the number of recipients of the RSVpreF vaccine is too small for researchers to ascertain even common serious adverse effects.”

SOURCES: and The New England Journal of Medicine, online June 22, 2022.

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