Stockholm and London both had the same coronavirus infection rates earlier in the summer, according to a new study, casting yet more doubt on the herd immunity theory.

Sweden's state epidemiologist Anders Tegnell, who is widely credited as the architect of the country's contentious response to the pandemic, justified Sweden's response by saying countries that imposed strict lockdowns would most likely suffer large second waves later in the year, whereas Sweden's would be smaller.

However, a study carried out by University College London academics and published in the Journal of the Royal Society of Medicine, estimated that the level of infection in Stockholm in April was actually around 17% — the same level as in London, according to analysis of tests carried out in England's capital in April and May.

The study poses a further challenge to herd immunity — the theory that a population will become immune to the coronavirus when at least 60% of people catch it.

There is currently insufficient evidence that coronavirus antibodies provide immunity to the virus.

It is not clear, howver, whether having antibodies offers total — or even partial — immunity to COVID-19, or how long such an effect may last.

A study by Kings College London published last month found that while 60% of people with the coronavirus had "potent" antibodies, just 17% had the same level of potency three months later.