Israeli Study Finds COVID Vaccine Window for Pregnant Women to Protect Newborns

Ido Efrati
Ido Efrati
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A pregnant woman being vaccinated against the coronavirus in Jerusalem in September.
A pregnant woman being vaccinated against the coronavirus in Jerusalem in September.Credit: Ohad Zwigenberg
Ido Efrati
Ido Efrati

Newborns are best protected from COVID-19 if their mothers get vaccinated between Week 27 and Week 31 of their pregnancies, an Israeli study has found. According to the study, vaccinating women during this period provides a higher level of antibodies in the newborns than it did for babies born to women who were vaccinated later in their pregnancies.

The 171 vaccinated women in the study were divided into two groups. The first was comprised of women inoculated between Weeks 27 and 31 of their pregnancies. The other group consisted of women vaccinated between Weeks 32 and 36. The women’s average age was 31.

The study demonstrated that antibody levels among newborns whose mothers were vaccinated later was 40 percent lower than those whose mothers were vaccinated in Weeks 27 to 31. In addition, the antibody levels of newborns whose mothers were vaccinated between Weeks 27 and 31 was twice as high as that of their mothers themselves. Among the babies whose mothers were vaccinated later, their antibody levels were the same as their mothers'.

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The findings from the study, which was carried out by the Hadassah University Medical Center at Ein Karem in Jerusalem and the Hebrew University medical school were published this month in the European journal Clinical Microbiology and Infection. The study was directed by Drs. Amihai Rottenstreich and Shay Porat and Prof. Dana Wolf.

“We estimate that the placenta is able to filter and select only the most efficient antibodies for neutralizing the virus,” Rottenstreich said. “We’ve seen it in the types of antibodies themselves and in tests of the levels of neutralization in practice."

Rottenstreich explained that “vaccinating pregnant women causes the antibodies created in the mother's body to be passed through the placenta to the fetus, thereby providing protection to newborns.”

This is particularly important, he said, because "COVID among newborns may lead to more serious illness than among older children. In addition, newborns can be another source for spreading the disease, and it's possible that antibodies that pass through the placenta can also assist in this context," he explained.

Infants younger than 6 months of age are not, at this stage, expected to be vaccinated against the coronavirus. Vaccination of children between the ages of 5 and 11 has just been approved in Israel and is to begin shortly. Clinical trials of the vaccine are currently under way for children from 6 months through 4 years of age.

“There is currently no study regarding babies younger than six months for safety reasons,” Rottenstreich said. "And actually, there's a 6-month period left during which babies are exposed to the disease, and among them the risk of serious illness is higher than among other groups of children, so there is all the more importance in the potential of protecting them via maternal vaccination during pregnancy."

A previous study by the same team of researchers that was published in April demonstrated that antibodies are passed from the mother to the fetus during the final trimester of the pregnancy. “The next step was to find out what the optimal time frame for vaccination protection was, both in terms of the mother’s antibody levels and the effect on the newborn,” Rottenstreich explained.

The research team is currently finalizing another study for publication, involving some 400 women, which examines the impact of vaccination timing through the entire pregnancy. But based on knowledge gleaned from this and other studies, it already appears that the optimal vaccination timing, when it comes to the benefit to the newborn, is the third trimester.

Rottenstreich cited the whooping cough vaccine as a comparable example. It is administered to women between the 27th and 36th weeks of their pregnancies to protect the newborns. “But the literature shows that the best protection is obtained from vaccines in the 27 to 31 week range,” he stressed.

Antibody levels among newborns whose mothers were vaccinated later was 40 percent lower than those whose mothers were vaccinated in Weeks 27 to 31.Credit: Valeriy Lebedev/Shutterstock

Rottenstreich explained that the transfer of antibodies to the fetus becomes effective toward the end of the second trimester, around Week 24. “When you vaccinate too early, the mother’s antibody level drops,” he said. “Even if antibodies are transferred to the fetus, they break up over time, and the resulting antibody level at birth is low.”

On the other hand, the window of opportunity for vaccination is also circumscribed at the other end, meaning that vaccinating too late in the pregnancy also undermines the effort to provide optimal protection to the newborn.

“The process of maternal antibody transfer to the fetus is continuous up to birth, and it takes time,” Rottenstreich explained. “In the first stage, it takes time for the mother to develop a high antibody count from the moment that she is vaccinated. At the second stage, the fetus needs to be exposed for a sufficient amount of time to a high maternal antibody count to build enough antibodies for maximum protection.”

The researchers believe that their study and follow-up studies will pave the way to the development of a better vaccination strategy for pregnant women – one that will both protect them and offer the best protection for their newborn, at least for the first months of the baby's life. That is when they cannot be vaccinated but still might be exposed to the virus.

The findings do not necessarily demonstrate that pregnant women should postpone getting vaccinated until the optimal window of time for the newborns, Rottenstreich noted. “But perhaps we should examine a recommendation to unvaccinated women to receive the first dose early in the pregnancy, and the second one between Weeks 27 and 31 of pregnancy,” he said.

Now the researchers are monitoring antibody levels among newborns to determine whether the newborns' immunization protection wanes over time and if so at what rate. That study is still ongoing.

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