Experts have urged caution over rising hopes that megacities struck hard by Covid-19 may be immune from a second wave of infections.
Blood-test studies conducted in London, Delhi, Moscow and New York – all of which have been badly hit by the pandemic – have indicated that some areas may be inching towards some kind of population-level immunity.
But virologists have warned that the studies, while promising, should be interpreted with caution and should not be used as an excuse for relaxing social distancing or hygiene measures. In short: that light is not necessarily a sign of the end of the tunnel.
The Indian megacities of Delhi and Mumbai are the latest to report positive findings from antibody studies. The Delhi government's health ministry sampled more than 20,000 of its residents at random between June 27 and July 10, with 23 per cent found to have Covid-19 antibodies.
In Mumbai, samples from more than 9500 people collected by two private labs showed that 24 per cent had produced antibodies.
And in New York earlier this month, antibody testing of people at an urgent-care clinic in the aptly named Corona neighbourhood of Queens showed that 68 per cent had positive results. At another clinic in Jackson Heights, 56 per cent tested positive.
City-wide test results have not hit the headlines in the same way – the latest regular antibody testing of around 2400 people in the New York City metropolitan area as a whole conducted by the US Centers for Disease Control showed antibody rates of 6.9 per cent.
New York, where there have been 227,000 infections and 23,000 deaths, has been particularly hard hit by the virus. Tightly packed districts with high levels of black and ethnic minority populations who have proven particularly susceptible to the disease are bound to see pockets of high antibody levels, says Dr Julian Tang, consultant virologist at the Leicester Royal Infirmary and honorary associate professor at the University of Leicester.
Tang says some areas may give a "high herd-immunity impression".
"You could have a bubble of higher-level herd immunity but it won't be fully protective. It might be 10 per cent in one post code and five per cent in another."
In the UK, testing of around 1000 samples per region taken from blood donors shows that 17.5 per cent tested positive for antibodies in London, compared to just under 5 per cent in the southwest of England.
Tang added that if parts of rural Scotland or Wales were showing antibody levels similar to London then that would be "much more interesting".
And he said that neither the Indian or New York studies could be extrapolated to the city-wide population as a whole. "We don't know where the gaps [in testing] are," Dr Tang said.
In Moscow a sample of 98,000 people conducted by the state authorities suggested 21.7 per cent had antibodies, and Mayor Sergei Sobyanin was particularly bullish about the results.
"A collective immunity has developed among the population in Moscow, which allows us to confidently leave the restrictions behind," he told state-run television channel Rossiya-24.
The Moscow sample is much bigger than the New York or Delhi ones but scientists caution that none of the most eye-catching studies have been published in peer reviewed journals. Nor have they revealed what antibody assays were being used.
A recent Lancet study of countries across Asia, Europe and North America found wide variations in levels of people testing positive for antibodies, ranging from 0.5 to 15 per cent.
And a study from Spain – which had a major outbreak – showed that just 5 per cent of the population had a positive antibody test.
In Sweden, which unlike its European neighbours did not pursue a lockdown strategy, random testing conducted by the country's Public Health Agency detected antibodies in 6.3 per cent of samples. This rose to 10 per cent in the capital, Stockholm.
Scientists believe that populations require about 60 per cent with antibodies to offer a degree of herd immunity. But protection can also come in the form of T cells, that the body gains from contracting similar viruses.
But one of the big questions, says Dr Jeremy Rossman, senior lecturer in virology at the University of Kent, is that no one knows what immunity to coronavirus looks like.
"If the numbers from New York were true and even if they were representative of the city as a whole we don't know if all those people have enough antibodies to prevent reinfection, if these antibodies are active enough and how long they last. Some of the data we have seen about antibodies shows that they look like they fade quite quickly," he said.
"Even if in a city like Delhi you got to 70 per cent of the population being immune, the remaining 30 per cent would be protected but not completely protected because there will be large population movement and mixing."
He added: "We all want a bit of hope right now but it sends out a dangerous message that we're getting close to being protected so we can lower our precautions."