A trial is under way in South Africa to test if the BCG vaccine against tuberculosis is also effective against coronavirus - something unproven at the moment.
Some 500 health workers are taking part in the study, which has seen 250 of them given the injection, and another 250 given a placebo.
Children immunised with BCG tend to suffer less from respiratory illnesses including asthma, Prof Andreas Diacon of Task, the clinical research organisation funding the study, told the AFP news agency.
"South Africa does vaccinate all newborns with BCG. Re-vaccinating adults could help reduce the consequences of this pandemic," Task said in a statement.
Covid-19 is a respiratory illness, but the World Health Organization (WHO) says there is no evidence that the BCG protects people from Covid-19 infection.
The health body said last month that two clinical trials were under way involving BCG, and when completed, their findings would be evaluated by the WHO.
Despite the lack of medical evidence, global search for the term "BCG" has spiked, according to Google.
Another concern for the WHO is that increased demand for the vaccine could mean there will be less of it available to inoculate children against tuberculosis.
Task in South Africa told AFP it planned to extend its BCG trial to up to 3,000 health-care workers in Cape Town, observing participants for at least a year.