Women who receive the mRNA COVID-19 vaccine during pregnancy pass high levels of antibodies to their babies, according to a study.
The effectiveness of COVID-19 vaccines lies in their ability to trigger the production of the right antibodies, blood proteins capable of protecting individuals from infection, researchers said.
Whether this protection could pass from mothers to their infants before birth had remained a question, they said.
The study, published in the American Journal of Obstetrics & Gynecology Maternal-Fetal Medicine on Wednesday, was conducted on 36 newborns whose mothers received either the Pfizer or Moderna COVID-19 vaccine during pregnancy.
The research team led by NYU Grossman School of Medicine in the US found that 100 per cent of the infants had protective antibodies at birth.
Antibodies can either be produced as part of the natural response to infection or triggered by vaccines.
Though the sample size is small, "it is encouraging that neonatal antibody levels are high if women are vaccinated," said Jennifer L Lighter, an associate professor at Hassenfeld Children's Hospital at NYU Langone, and the study's senior author.
The team was able to tell apart antibodies in the neonatal blood that were created in response to natural infection from those made in response to the vaccines.
The researchers noted that the result is relevant because natural antibody responses against the SARS-CoV-2 virus, that causes COVID-19, are not sufficiently protective for many people.
Recent data from the US Centers for Disease Control and Prevention (CDC) suggests that just 23 per cent of pregnant women have been vaccinated, despite growing evidence of prenatal vaccine safety.
The researchers observed the highest levels of antibodies in cord blood of mothers who were fully vaccinated during the second half of their pregnancies.
This insight provides evidence of transferred immunity to neonates, which correlates to protection against infection for infants during the first months of life, they said.
"Studies continue to reinforce the importance of vaccines during pregnancy and their power to protect two lives at once by preventing severe illness in both mothers and babies," said Ashley S Roman, Professor at NYU Langone Health, and one of the study's principal investigators.
"If babies could be born with antibodies, it could protect them in the first several months of their lives, when they are most vulnerable, Roman said.
Roman and colleagues confirmed the strong evidence that the mRNA vaccines are safe during pregnancy in a study published on August 16 in the same journal.
That study found no increased risks during pregnancy, birth complications, or identifiable risks to the fetus among those who received the vaccine.
Existing studies analyse antibodies to the spike protein (anti-S IgG) alone, which may be present after natural infection or vaccination, and do not include antibodies to the nucleocapsid protein (anti-N IgG), which is only present following natural infection.
Among the 36 samples collected, all had high levels of anti-S IgG. Of those samples, 31 were tested for anti-N IgG and were negative.
"High levels of transplacental antibody transfer are not surprising. It is consistent with what we see with other immunizations," Lighter said.
"Our findings add to a growing list of important reasons why women should be advised to receive the COVID-19 vaccine during pregnancy for the added benefit of their newborn receiving crucial protection," he said.
The researchers emphasised that more study is needed to determine how effective the infant antibodies are, how long protection will last, and if vaccination in the second half of pregnancy may confer higher levels of antibody transfer than vaccination earlier in pregnancy.