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Pfizer’s vaccine to protect newborns from respiratory syncytial virus, or RSV, by vaccinating their mothers late in pregnancy reduces the risk that babies will need to see a doctor or be admitted to hospital with a moderate infection to severe before 6 months of age, according to a new analysis by government regulators.
Many parents were eagerly awaiting this news, particularly after last year’s long and severe RSV season.
RSV is one of the leading causes of hospitalization in infants and the elderly each year, and it typically hits the hardest during the winter months.
Scientists have been working on an effective RSV for young children for about 60 years, so there’s a lot of excitement around the possibility of a candidate getting that close to the finish line.
But the news is not all rosy. Safety data published in a agency analysis Tuesday also showed a slightly higher proportion of preterm births in babies whose mothers received the experimental RSV vaccine compared to those who received a placebo: 5.7% vs. 4.7%, respectively.
The difference between the groups was not statistically significant, meaning it could be due to chance. The preterm birth rates in both groups were lower than the preterm birth rates reported in the general population, which is around 10%. And most babies, even when they were born prematurely, were born after 34 weeks of pregnancy, just a few weeks before they were due.
Still, this finding will likely spur discussion this week at a meeting of a group of independent experts advising the US Food and Drug Administration on its vaccine decisions.
Late-stage trials of a similar maternal RSV vaccine being tested by pharmaceutical company GSK were halted last year over safety concerns.
GSK later reported that there was an increased risk of preterm birth among the vaccinated mothers in their study compared to those who received a placebo. There were 238 preterm deliveries among 3,496 vaccinated mothers (6.8%) compared with 86 among 1,739 pregnant women (4.9%) in the placebo group. There were also 13 neonatal deaths in the vaccine group, compared with three in the placebo group, according to a recent report in the BMJ.
The FDA’s Vaccines and Related Biologicals Advisory Committee, or VRBPAC, will meet Thursday and vote on whether to recommend approval of Pfizer’s maternal RSV vaccine.
In terms of efficacy, the FDA reviewers said the vaccine was most effective for the youngest infants and that its protection waned over time.
During the first three months after birth, the vaccine was 82% effective in preventing severe RSV disease and 57% effective in preventing babies from needing to see a doctor for RSV infection. At six months after birth, the vaccine was 69% effective in preventing severe RSV disease and 51% effective in preventing a visit to the doctor for RSV-related respiratory problems.
After six months, the vaccine was as effective as placebo in preventing RSV that became severe enough to send babies to the doctor’s office.
For the most part, the vaccine appeared to be safe, with balanced serious adverse events and reactions between the vaccine and placebo groups.
There were 17 deaths in the main study: five in the group that received the vaccine and 12 in the group whose mothers received a placebo.
The FDA agreed with Pfizer that four of the five deaths in the immunized group were likely unrelated to the vaccine. The agency said it could not exclude the possibility that one death, in a baby born very prematurely 10 days after her mother was vaccinated, was related to the vaccine.
One baby in the placebo group died of an RSV infection.
Pfizer’s RSV vaccine is made with a key piece of the virus: its F proteins, the part of the virus that attaches to and infects cells. The vaccine uses F proteins from the two main subgroups of the virus, A and B, making it a bivalent vaccine.
The vaccine tested in pregnant women has the same dose and structure as the one the company is testing for adults 60 and older.
The FDA has reviewed Pfizer’s vaccine for seniors and is due to make a decision on whether to approve it by the end of the month.
If approved, it will become the second RSV aimed at older adults. The agency recently approved an RSV vaccine for the elderly made by GSK.
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