The government has said the final stage of reopening is dependent on the evolution of the virus.

The number of U.K. cases of a worrying India-dominant coronavirus variant more than doubled for a second week as authorities also monitor a new mutation of the virus, adding fresh doubt to U.K. plans to fully unlock the economy.

Health officials have now detected 3,424 cases of the B1.617.2 India-dominant variant, Public Health England said Thursday in a statement. That’s up from 1,313 last week, and 520 a week earlier. They’re also investigating a separate variant with an “unusual mutation profile”, with 49 cases logged so far — mainly in Yorkshire, northeast England.

Scientists at PHE say there is currently no evidence to suggest vaccines will be less effective in protecting people against severe illness from the India-dominant variant, but believe it could be more transmissible than the fast-spreading Kent variant that led to the country’s third lockdown in January.

Prime Minister Boris Johnson said last week that he was “anxious” about the India-dominant variant, also known as VOC-21APR-02, and warned that it could affect the country’s plans to reopen the remaining bits of the economy from lockdown on June 21.

Those concerns prompted authorities to alter the country’s vaccination program, shortening the interval between doses. And even though stage 3 of the government’s reopening plan went ahead Monday, the fast spread of the new variant remain a cause for concern.

We have updated the number of confirmed cases of #COVID19#variants identified in UK.

More: https://t.co/NxiBAaLVss

Cases of VOC-21APR-02 have risen by 2,111 since last week. These are still predominantly affecting the North West of England – particularly Bolton – and London. pic.twitter.com/gADvCktEev

— Public Health England (@PHE_uk) May 20, 2021

“As we start to get back to normal life, it is very important that we do not become complacent,” Health Security Agency Chief Executive Officer Jenny Harries said in PHE’s statement. “All of us need to remain responsible and vigilant.”

Nevertheless, ministers and officials this week have struck an increasingly optimistic tone, with the premier saying Wednesday that he has “increasing confidence” vaccines are working against all coronavirus variants. More than 70% of U.K. adults have now had at least one dose of vaccine, making the country’s program one of the most advanced in the world.

Justice Secretary Robert Buckland told Sky News on Friday that caution was the “watchword” when it came to reopening the economy further. He said that even if social distancing rules were lifted as planned, he would continue to wear a mask if he had signs of a cold.

Most of the 49 cases of the new variant under investigation are in Yorkshire and the Humber, in Northern England, according to PHE. “There is currently no evidence that this variant causes more severe disease or renders the vaccines currently deployed any less effective,” it said.

On Monday, international holidays were allowed again, and pubs and restaurants were allowed to serve customers indoors. The government has said the final stage of reopening — which will be June 21 at the earliest, is dependent on the evolution of the virus.