COVID-19: The three days in April that may have fuelled UK outbreak of Indian coronavirus variant

If the new Indian variant does install itself as the main variant of COVID-19 in this country; if it does lead to more cases and in turn more deaths - and both of those remain big ifs - the question of how this happened is likely to focus on three days in April.

And the spotlight will likely fall not just on the scientists advising the prime minister, but on Boris Johnson himself.

For the decision to delay putting India on the red list of countries, from which travel is heavily limited, and the decision to implement this not immediately but with a gap of just over three days - during which thousands of travellers from India entered the country amid a surge of demand for flights - happened in the shadow of one of the biggest of all political and economic stories of recent decades: Brexit.

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One of the overarching ambitions of this country since leaving the European Union and ending the transition period at the end of last year, has been to seal as many trade deals as possible with as many of the world's leading economies.

With the arrival of Joe Biden in the White House, ambitions of agreeing a trade deal with the US any time soon were scaled back (the working ambition is now "at some point before the US mid-term elections") and attention swung to other major economies.

India has long been a promising target for those at the Department for International Trade.

It is not just former colonial ties which make it attractive: Indian companies are now among the biggest investors in the UK and Britain has something of a trump card in these talks: visas.

The Indian government has often sought to increase the number of visas available to Indian citizens to travel, work and study in the UK. Any travel restrictions remain a sore point. There are other low-hanging fruit too, including a long-standing dispute over Scotch whisky which the EU's negotiators have failed to resolve in recent years.

Sealing a deal, even a provisional one, with one of the world's fastest growing and dynamic economies, has long been a goal for the prime minister.

The fact that he might be able to declare victory in the battle over Scotch, and the tantalising prospect of agreeing a deal before the EU - which is also in parallel trade discussions with India - only added to the allure.

All of which is why Mr Johnson had been so determined to make India the destination for his first major foreign visit as prime minister. The trip had originally been slated for January, but was delayed as the UK faced a sharp increase in COVID cases.

It was rearranged for late April, with Mr Johnson due to fly out for meetings and negotiations on April 25.

The working plan was that Mr Johnson would be able to announce that early discussions were now under way about a deal - and that formal negotiations would begin in the autumn. There would be talk of more visas for Indian migrants and of resolving the long-standing impasse on Scotch.

It was to be one of the early "wins" for the PM as he sought to underline the economic opportunities that lay outside the EU.

Yet as the date of the visit approached, the epidemiological data coming out of the Indian subcontinent began to deteriorate. Cases of COVID-19 had been rising fast throughout March, causing concern amid the global public health community.

Data on cases and deaths in India has never been as reliable as the numbers in Europe, with many epidemiologists suspecting vast undercounting of infections and deaths both last year and this. But even this likely undercounted data had begun to show a significant uptick in cases by late March.

By 2 April there was enough disquiet that the UK added the two countries neighbouring India on its east and west, Pakistan and Bangladesh, to its "red list". Foreign travellers from countries on the list cannot travel to this country; UK and Irish citizens and residents can enter, but must stay in a government-assigned hotel for a 10 day quarantine period.

The goal of this policy is to prevent the entry to the country of any dangerous variants of the disease - and the South African and Brazilian variants were known to be circulating in these countries.

Yet even as Bangladesh and Pakistan were added to the red list (the implementation took place on 9 April), questions were being asked about why India was not joining them.

In early April there were stories about the country's cemeteries being overwhelmed. In the days following 2 April the number of new cases of COVID-19 rose beyond an average of 100,000 a day, and then over 200,000 a day. Still India remained off the red list.

It is at this period that the UK started to detect an influx of positive COVID-19 cases from India. According to data from Public Health England, of the 3,345 people arriving from India between 25 March and 7 April, 4.8% tested positive for COVID-19. At that stage, the percentage of people in England with COVID-19 was 0.1%.

It was also at this stage that Public Health England began to pick up arrivals of three Indian variants around the UK.

In particular, the most worrying of all those variants, B1.617.2, which is the variant which is spreading most quickly and has now claimed at least four lives, was first detected in tests carried out on travellers arriving from India on the week ending 29 March.

According to PHE data, at least 122 passengers arriving from Delhi and Mumbai between late March and 26 April were carrying this variant, now designated a "variant of concern". All but a handful of these travellers would have been allowed, under the rules then in place, to leave the airport and travel home, where they were asked to self-isolate.

Even as cases of the new variant were arriving in the UK, concern was growing in Whitehall about why India had still been left off the red list. There is little publicly released data or methodology on most of these decisions, which are technically in the hands of the Joint Biosecurity Centre.

It says it considers a variety of factors, including the prevalence of the disease in given countries and the quality of the infrastructure there. During this period many in the epidemiological community voiced concern about the omission. Some wondered why the government was taking so long.

Two weeks on from the decision to put Pakistan and Bangladesh on the list, there came an answer of sorts.

On the morning of 19 April, Downing Street announced that the prime minister's trip to India was cancelled. A few hours after news of the cancellation of the prime ministerial visit, Health Secretary Matt Hancock told the House of Commons that India would also be added to the red list.

By then - the afternoon of 19 April - the daily number of new cases in India had surpassed a quarter of a million. Within a couple of days the official numbers - themselves widely believed to be an undercount of reality - would mean this was officially the biggest outbreak in any country during the entire pandemic.

However, the UK's decision to place India on the red list was not immediate. Instead, three full days and nights would go by before it would be implemented.

These delays are not unusual during the short history of COVID travel restrictions. Invariably when a country is added to the list it is given a period of time - often up to a week - for travellers to make the necessary plans in advance.

However, there is nothing to stop ministers imposing these restrictions far sooner. Indeed, when the hotel quarantine scheme was first announced, Downing Street briefed journalists that countries could be added to the list "at a few hours' notice". That did not happen with India.

In the following three days demand for flights between India and the UK shot through the roof.

Travel website Skyscanner reported a 250% leap in searches for flights from India to the UK. There are typically 30 such flights a week.

In those days, four airlines requested to operate an extra eight flights from India due to the surge in demand ahead of the implementation of the hotel quarantine. The requests were turned down, but thousands of passengers nonetheless travelled into the UK.

Even before this three-day period, the proportion of cases of B 1.617.2 imported from India had been on the rise. But between 4 April and 2 May, this variant rose from 4.9% of all cases detected among travellers, to 40.9%.

The single biggest increase in these weekly numbers was the week which included the three and a half days between the afternoon of 19 April and the early morning of 23 April.

It is worth underlining that it is still much too early to say whether the B 1.617.2 will indeed change the course of the pandemic in the UK. It is certainly spreading faster than any other variant of concern since the famous Kent variant which established itself as the dominant strain of the virus in the winter.

However it remains a small fraction of the total of cases, which are themselves small in comparison with recent months.

As of 5 May, the percentage of people in England with any variant of COVID-19 had dropped to just 0.07%, the lowest level since early September, according to data from the Office for National Statistics. Hospitalisations, deaths and case numbers remain low.

However, cases are growing fast in a few areas where the Indian variant seems to have established itself, including Bolton, Blackburn and Leicester. By contrast, a cluster of cases in London seems to be under control.

It is too early to tell whether this presages the beginning of another spread throughout the country.

However, one factor is decisively different from the winter or indeed last year: the majority of UK citizens have now received a first dose of a vaccine, and the early evidence suggests, tentatively, that these vaccines provide adequate protection against this new variant.

Outside of India, there are few countries other than the UK that have quite so many confirmed cases of B1.617.2 - though this may owe itself partly to the fact that this country carries out more gene sequencing than any other country.

Even so, if the Indian variant establishes itself as the dominant strain in the UK, jeopardising the sacrifices and suffering during a third period of lockdown, the prime minister will come under increased scrutiny to answer why the decision was left so late to impose restrictions on travel from India, why travellers were given an extra three and a half days to come to the UK and why the rationale on which country is on or off these travel lists remains so murky.