Dr Tedros Adhanom Ghebreyesus said that the emergence of mutated versions of the virus had "raised questions" about their impact on vaccines particularly after South Africa decided to halt AstraZeneca vaccinations when a study showed the vaccine was less effective at preventing the variant there.
"This is clearly concerning news," said Dr Tedros at a press conference on Monday, while emphasising that there were "important caveats" to the vaccine study such as the younger age of participants.
It remains to be seen whether the AstraZeneca/Oxford COVID vaccine prevents severe disease in those infected with the South African variant.
But, Dr Tedros warned, it serves as a reminder that people need to reduce spread of the virus through physical distancing and hand washing.
"Every time you decide to stay at home, to avoid crowds, to wear a mask, or to clean your hands you're denying the virus the opportunity to spread and the opportunity to change in ways that could make vaccines less effective," he said.
Experts have said that although the coronavirus does not mutate as frequently as the flu, the more it spreads, the more opportunities it has to mutate.
But he compared the COVID vaccines to flu vaccines nonetheless, stating that future jabs and boosters would have to be adjusted.
"This is what happens with flu vaccines which are updated twice a year to match the dominant strains," Dr Tedros said.