"This is a clearly more transmissible virus, this [variant] which originated in India. The vaccine may be less effective against mild disease, but we don't think they are less effective against severe disease," Harnden told BBC Radio 4.
The JVCI believes the vaccines are "almost certainly less effective against transmission," he added.
According to Harnden, the committee expects that the virus can be better prevented from spreading by vaccinating those in high risk groups and bringing forth the second vaccination dose to people over the age of 50 by four weeks.
"If we immunize 18-29-year-olds, for instance in these areas, we are taking vaccines from somebody else in the country," the official said, adding that "Immunity takes a number of weeks to develop so it is not a very good strategy for preventing transmission, what we want to do is prevent disease."
Harnden admitted that while the Indian variant is "certainly more transmissible than the major variant that is circulating in the UK," it is "social factors" that also have a role to play in the transmission of the virus. The official also warned that UK hospitalization rates can still go back to growth.
"Those unvaccinated, vulnerable individuals, those over 50 particularly, and in all our priority groups who are unvaccinated, will develop COVID-19 from this transmissible virus and will end up in hospital and that's our real worry," Harnden said.
Prime Minister Boris Johnson said on Friday that the United Kingdom should accelerate its COVID-19 vaccination program to contain the fast spread of the Indian variant, which has now formed clusters in various parts of the country.