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Health experts stress that cases of fully vaccinated individuals becoming infected with Covid-19, commonly called "breakthrough infections", do not imply that the vaccines are ineffective.

"The risk (of infection) among vaccinated people should be closer to 0.0 per cent everywhere, but the fact that they are not is less a matter of some failure of the vaccines themselves than the fact that the virus is spreading so widely," Dr James Hamblin, a public health policy lecturer at the Yale School of Public Health, wrote in his online health bulletin on Friday.

"The more people who refuse to get vaccinated, the more vaccinated people will get infected."

Separately, the preventative medicine physician wrote on Twitter: "No one... ever claimed vaccines will stop the virus from landing on you. Vaccines prevent serious illness. They do that very well."

Transmission risks

What about the chances of fully vaccinated people infecting others around them with the virus? The jury is still out on this.

In a study last month, Chinese researchers reported they had found early evidence that while those who are vaccinated and infected with the Delta strain do not necessarily get sicker, they do become more contagious and for longer.

The researchers said those infected with the Delta variant had on average about 1,000 times more virus in their respiratory tracts than people who caught the original strain.