The Centers for Disease Control and Prevention had in December advised the public to steer clear of the vaccine, which was authorised as a single shot.

The study, sponsored by the vaccine developer, was conducted between Jan 1 and Sept 7 last year, before the Omicron variant was discovered.

Around half a million South African health staff have received Johnson jabs as part of clinical trials.

The single shot J&J vaccine was never as effective as its double-shot mRNA counterparts, but some evidence suggests it is less susceptible to waning than Pfizer and Moderna, because of the viral vector technology behind it.

A second shot given 56 days after the first provided 75% protection against symptomatic Covid in a global clinical trial, with that level rising to 94% in the US.

In two clinical studies that have not yet been peer-reviewed, the additional shot evoked a nine-fold increase in antibodies to the coronavirus.

The new warning label for caregivers and recipients says in most people, symptoms began within 42 days of receiving the vaccine, and 'the chance of having this occur is very low'.

It says the immune response lasts at least eight months.

This comes as several countries limit the use of jabs like AstraZeneca and Johnson & Johnson and opt instead for vaccines like Pfizer and Moderna.

He says this will eliminate the need to make a second trip in the rural areas.

The CanSino and Johnson & Johnson jabs have been given conditional registration approval while the Pfizer vaccine has been given the green light for use on children 12 and above.

Lagging behind many other countries, South Africa has only vaccinated just over 1% of its population.