Johnson & Johnson's also-ran COVID-19 vaccine has been cleared in the EU as a booster shot, although its role in the US could be impacted by new data on rare blood clots.
The UK government has said it will step up its COVID-19 booster vaccination dramatically, with everyone in England aged over 18 offered a third dose before the end of the month.
Preliminary study results from South Africa have suggested that antibodies stimulated by Pfizer and BioNTech's Comirnaty are less effective against the new COVID-19 variant Omicron.
The Omicron variant could supersede other strains of COVID-19 within the next few weeks to become dominant in the UK, according to an infectious diseases expert.
A trial of booster vaccinations to protect against COVID-19 has shown that the jab significantly strengthens the body's T-cell immunity against the disease.