One thing that we’ve learned during the Covid-19 pandemic is that pregnant women are at higher risk for severe disease. Also, the chances of having negative outcomes for mother and baby—such as preeclampsia, preterm birth, and stillbirth—increase with infection. However, when the Covid-19 vaccines were first released, many expectant mothers were hesitant to get them since pregnant women weren’t included in the clinical trials.

Since then, thousands of people have been vaccinated, including pregnant women. Subsequent studies show that getting vaccinated during pregnancy is safe and protective for the mother and the baby. One study published last month found that babies with Covid antibodies from their mother being vaccinated during pregnancy had a 69% lower chance of being hospitalized with the disease in their first 6 months of life.

While research shows that receiving the vaccine during any trimester provides benefit, there may be ways to optimize protection based on when the shots are given. Researchers from Massachusetts General Hospital and Brigham and Women’s Hospital analyzed the antibodies of 158 women who received the Covid-19 vaccine during their pregnancies. The women had either Pfizer-BioNTech’s or Moderna’s vaccine. Those who got the vaccine during either the first or third trimesters had the strongest immune responses. The study authors state it may be due to pregnant women’s immune systems being more active during the first and third trimesters.

The study also found that the transfer of antibodies from mother to baby was most efficient during the first and third trimesters. However, the absolute number of antibodies transferred was highest if the mother was vaccinated during the second or third trimester. Using this evidence, researchers recommend that pregnant women get vaccinated or receive a booster during their third trimester to provide their babies with the highest level of protection after birth.

Vaccine makers will release new boosters this fall designed specifically to combat the omicron subvariants, BA.4 and BA.5, which are the primary strains increasing throughout the country. The Food and Drug Administration (FDA) advised the companies to create a two-part vaccine for a fall booster campaign. One part of the vaccine will be the original formula, and the other will be based on the BA.4 and BA.5 omicron subvariants. Data from mice show that a vaccine based on this formula generates a stronger immune response.

Peter Marks, director of FDA’s Center for Biologics Evaluation and Research, stated, “As we move into the fall and winter, it is critical that we have safe and effective vaccine boosters that can provide protection against circulating and emerging variants to prevent the most severe consequences of covid-19.”