It could be another "two to three weeks" before it’s clear whether the final step in lifting lockdown can safely go ahead, according to a former senior scientific adviser to the government.
Professor Neil Ferguson, an epidemiologist at Imperial College London, told a briefing for the German media that the precise biological advantage of the variant over previous versions of the virus remains uncertain.
If it is only 20-30% more transmissible and has a small impact on the effectiveness of the vaccine, it would cause a significantly smaller third wave of infections than had been initially feared, he said.
Even before the B.1.617.2 variant was classed as a concern, models showed that COVID cases would rise over the summer and autumn as a result of lockdown being lifted on 21 June.
The expectation was that hospital admissions wouldn’t rise so fast because most vulnerable people would have had two doses of vaccine.
But speaking at an event organised by the Science Media Centre in Germany, Prof Ferguson said it’s too soon to be sure what impact the Indian variant will have on the numbers.
"It’s not how high cases rise, but how quickly," he said.
"If they double every 10-14 days and hospital admissions follow the same trend, then there is a concern. We were expecting cases to rise as we relaxed restrictions but if they rise too quickly that’s a problem."
Prof Ferguson said the UK is in a much better position now than when the Kent variant emerged in December. Infection levels are low and vaccination rates are high, which will help to control the virus.
Data from Singapore suggests the Indian variant is more common in children.
Prof Ferguson said there were "signals" in the UK that people under 21 are more likely to be infected with the variant than other versions of the virus.
But he added uncertainty remains over whether that is due to a biological enhancement from its mutations or the "founder effect", whereby the virus was brought back by travellers, who passed it to children, who then took it into local schools.