Passive Exposure to Oral Polio Vaccine May Protect Against COVID-19

NEW YORK (Reuters Health) – Mothers of children immunized with the oral polio vaccine (OPV), which is made from a live attenuated virus, seemed to be protected against symptomatic COVID-19 for at least six months in a longitudinal cohort study in Iran.

“Attenuated polioviruses are shed in large quantities in stool of children immunized with OPV, and mothers get exposed to it while changing diapers and other close contacts with their babies,” Dr. Robert Gallo of the Institute of Human Virology, University of Maryland School of Medicine in Baltimore and the Global Virus Network told Reuters Health by email. “Therefore, they are passively vaccinated with live virus vaccine that stimulates their innate immune system.”

“This study provides more support for the concept that innate immunity can be utilized to help protect against new virus infections, particularly when there is no standard vaccine available,” he noted.

“For COVID-19, the vaccine antibody response is similar to flu and tends to fade after 5 or 6 months and require boosting,” he said. If boosters aren’t available or someone’s immune response to vaccination is fading, “this approach of using live attenuated vaccines to stimulate innate immunity is a good one,” he suggested.

As reported in JAMA Network Open, from a cohort of close to 88,000 women, Dr. Gallo and colleagues analyzed data from 419 mothers (mean age, 35.5) indirectly exposed to OPV and 3,771 age-matched women with no exposure to OPV from March – December 2020.

During the nine-month study, COVID-19 was diagnosed in 1,319 of the overall study cohort population (incidence of 151 per 10,000).

No mothers whose children received OPV developed COVID-19 after a median follow-up of 141 days; 28 women (0.74%) in the unexposed group were diagnosed with COVID-19.

Point-by-point comparison of the survival curves of the exposed and unexposed groups showed that indirect exposure to OPV was significantly associated with decreased COVID-19 infection; at nine months, the probability of remaining without infection was 1.000 in the exposed group versus 0.993 in the unexposed group.

Of note, a study limitation acknowledged by the authors is that the mothers in the OPV group who were caring for an infant or young child might have spent more time at home and hence had less exposure to SARS-CoV-2. Conversely, some mothers might have been at higher levels of exposure because as part of Iranian ceremonies, they may have been visited by numerous friends and relatives who came to meet the newborn.

The authors conclude, “Further study of the potential protective effect of OPV should be conducted, especially in nations where OPV is already in use for polio prevention and specific COVID-19 vaccines are delayed, less affordable, or fail to meet demand.”

Dr. Anna Bershteyn, an assistant professor in the Department of Population Health at NYU Langone Health in New York City commented in an email to Reuters Health, “This study raises the question of whether mothers might benefit from their child’s polio vaccine by having lower SARS-CoV-2 risk. A rigorous study would be needed to confirm if this is true. This study was observational and did not control for important factors such as the baby’s age. The social lives of mothers change a lot through the ages and stages.”

“Additionally,” she said, “very few of the mothers were ever diagnosed with COVID-19 in the study: Only 28 out of more than 4,000 mothers were followed throughout 2020. This could indicate low rates of testing, or a study population that was at very low risk of COVID-19.”

SOURCE: JAMA Network Open, online November 24, 2021.

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