Do cloth facemasks reduce the risk of illness during the COVID-19 pandemic?
Beneficial non-specific effects of the live Oral Polio Vaccine
The Oral Polio Vaccine (OPV) has been in use since the late 1950s, and its effects on preventing polio infection are well known. What has been studied less, is the effect of OPV on other health threats.
The effects of OPV beyond the prevention of polio, have mostly been studied in children, where OPV appears to lower the risk of dying, prevent respiratory infections, and bacterial diarrhoeas – especially for boys.
There is also some evidence that OPV has a broader impact on health in adults: In a study conducted in 1970, > 40,000 factory workers given OPV had approximately half the risk of airway infections compared with >18,000 not-OPV vaccinated factory workers. This finding was, however, not widely pursued.
Oral Polio Vaccine and COVID-19
The COVID-19 pandemic changed priorities. In a situation without tools to combat the new virus, evaluating whether older drugs (including vaccines) could be effective against COVID-19 infection or severity became important. One of these tools is OPV.
Based on observational data indicating lower risks of COVID-19 in individuals exposed to OPV, researchers have argued that OPV could reduce the risk of COVID-19.
In May 2022, the results of a randomised trial supported that OPV may provide partial protection against COVID-19. In this trial, 1115 adult volunteers in Kirov, Russia, were randomised to a single dose of OPV or placebo and followed for 3 months. During 3 months of follow-up, a significantly lower number of laboratory-confirmed COVID-19 were detected in the OPV group than in the placebo group (25 vs. 44 cases, test of no difference: p=0.04). There was no difference in the number of self-reported cases of respiratory infection between the two trial arms.
The Bandim OPV-COVID trial
Before vaccines against COVID-19 were available, we investigated whether OPV could reduce the risk of illness and mortality during the pandemic in Guinea-Bissau. We cluster-randomised houses in the urban Health and Demographic Surveillance System (HDSS) of the Bandim Health Project to either an intervention or control arm. We then invited adults above the age of 50 years living in the houses to participate in the trial. Participants living in the intervention houses received a single dose of bivalent OPV, while participants in control houses received no vaccine. Between July 2020 and April 2021, we enrolled 3729 participants aged 50+.
During the 6 months of follow-up, we collected information on consultations through health facility records, hospital admission through telephone interviews, and registered deaths through the HDSS from a total of 3726 participants. Through monthly telephone calls, we also collected information about symptoms of infections.
Map of the urban HDSS study area in Bissau, Guinea-Bissau
Overall, the rates of infectious illness and death combined were similar in the OPV (intervention) and no-OPV (control) group (3% (-18-21%) lower in OPV-arm). However, similar to earlier observations in young children, the beneficial effect of OPV was stronger for males than females. Men who received OPV had a 29% (2-49%) lower risk of illness/death than men receiving no OPV. At the same time, there was no benefit of OPV in women. The sex-differential effect was observed both for the composite outcome and the separate components: death, infectious admission and consultations for infections. At the same time, we observed that 10% (3-17%) more OPV-recipients than non-recipients reported symptoms of infections during the telephone interviews with no difference between males and females. This could potentially be because OPV stimulates the immune system broadly, making both men and women respond more vigorously to infections.
Thus, OPV may have provided partial protection against COVID-19 and protected men against morbidity and mortality during the pandemic. This emphasises the importance of assessing the real-life effect of interventions – both new and old ones, to be better equipped for future pandemics