Two doses of mRNA COVID-19 vaccine during pregnancy are highly effective against delta and moderately effective against omicron infection, Canadian researchers report today.
The vaccines are also linked to a reduced level of babies under six months requiring hospital treatment
The study, led by the University of Toronto and published in the latest edition of *The BMJ*, found the greatest protection against omicron infection was when a pregnant woman receives a second vaccine dose in the later stages of pregnancy.
This also conferred the greatest protection for infants in their first eight weeks of life.
Although COVID vaccines are not yet licensed for infants under six months of age, there is growing evidence that vaccination during pregnancy may reduce the risk of infection and hospital admission in infants as antibodies are transferred in passive immunity.
The research team analysed data from infants younger than six months of age who were born in Ontario between 7 May 2021 and 31 March 2022 and who were PCR tested for COVID-19 infection between 7 May 2021 and 5 September 2022.
These data were linked to the mother’s mRNA COVID-19 vaccination status during pregnancy (one, two or three doses) or unvaccinated (no doses), and to recorded delta and omicron-related hospital admissions in infants.
In total, 8,809 infants were included in the main analysis, of whom there were 99 positive delta cases compared with 4,365 negative controls, and 1,501 omicron cases compared with 4,847 controls.
Two vaccine doses during pregnancy were 95% effective against delta infection in infants and 97% effective against infant hospital admission due to delta.
Effectiveness of two doses against omicron infection in infants and admission to hospital was 45% and 53%, respectively, but improved with a third dose during pregnancy at 73% and 80%, respectively.
They found the effectiveness of two doses against omicron infection was highest when the second dose was given in the third trimester of pregnancy compared with the first or second trimesters T 53%, 47% and 37%, respectively.
Effectiveness of two doses against omicron infection waned over time, from 57% between birth and eight weeks to 40% after 16 weeks of age.
As well as the 8,809 infants studied in the main analysis, they also found 421 infants who were born to mothers who received only their first vaccine dose during pregnancy. This conferred less protection against infection in infants than two or three doses (81% against delta and 30% against omicron).
Although this is an observational study, the researchers say they used detailed information on hospital deliveries, mother’s vaccination status, and PCR test results. Because their findings were similar after further analyses, the results are robust, they say.
“In this study, we show that maternal COVID-19 vaccination during pregnancy might have dual benefits by also conferring protection to their infants,” they conclude.
Jorgensen SCJ, Hernandez A, Fell DB et al. Maternal mRNA COVID-19 vaccination during pregnancy and delta or omicron infection or hospital admission in infants: test negative design study. *BMJ* 9 February 2023; doi: 10.1136/bmj-2022-074035
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