Professor Andrew Hayward urged Boris Johnson to “err on the side of caution” – amid criticism that he is refusing to act to avoid having to cancel his own trade trip to the country next week.
The B.1.617 strain has been linked to a dramatic surge in infections which is wreaking havoc across India, with evidence it may be capable of evading antibodies and T-cells.
There have been 77 confirmed cases in the UK, but India – unlike Pakistan – is not on the ‘red list’ of countries from which travellers must enter 10 days of hotel quarantine.
Prof Hayward, professor of infectious disease epidemiology at University College London, pointed out that Hong Kong had imposed a two-week travel ban on India.
“There are number of concerning features about this – both in terms of mutations that this variant carries as well as the epidemiological picture overall in India,” he told BBC Radio 4.
“The evidence of increased transmission and escape from immunity is circumstantial. That said, it is going to take a number of weeks at least before that evidence gets firmed up and we find out more.”
He acknowledged the “harm” from restrictions on India – with both families and economies closely linked – but added: “On the other hand, what we have is an unknown level of risk.
“My own preference in all of this is to err on the side of caution and to act sooner rather than later. But, ultimately, that is going to be a political decision.”
Across India, there have been reports of a shortage of hospital beds, life-saving drugs and medical oxygen.
But Dr Susan Hopkins, the chief medical adviser for NHS Test and Trace, has said there is not yet enough data to classify the new Indian strain as a “variant of concern”.
“We have seen a couple of cases [of the Indian variant] that haven’t arisen from travel, but we’re still trying to look in great detail at where they might have acquired it from,” she said on Sunday.
No 10 has insisted the trade trip – already curtailed to as little as one day, from a planned four – will “prioritise the safety of those involved and all elements will be Covid secure”.
But Labour has joined calls for it to be scrapped, after members of the Indian cabinet reportedly tested positive for coronavirus and with business leaders reluctant to travel for meetings.
George Eustice, the environment secretary, defended the government’s decision not to further restrict travel to India, but said the situation was being kept “under regular review”.
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