Sweden's state epidemiologist has said the country has been "surprisingly slow" at achieving immunity.
The death toll in Sweden has just passed 5000 and the Scandinavian nation has one of the highest mortality rates in the world with a per capita death rate of 487 per 1 million people - about 10 times higher than neighbouring Norway.
Yet an analysis by Werlabs AB of 50,000 tests showed that only 14 per cent of those living in the Stockholm region tested positive for coronavirus antibodies.
In Bergamo, considered to be the centre of Italy's deadly outbreak, about 57 per cent of people had antibodies.
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Sir Patrick Vallance, the UK's chief scientific adviser estimated that 60 per cent of people would need to be infected to achieve herd immunity from the novel coronavirus.
Sweden chose not to implement a strict lockdown and schools, shops and restaurants have all remained open during the course of 2020.
Instead, the government deferred to scientific advice and recommended Swedes observe social distancing.
There had been expectations this would have allowed Sweden to achieve herd immunity without imposing an economically and socially crippling lockdown.
However, in an interview with Swedish Radio, Anders Tegnell, the country's chief epidemiologist, admitted that immunity rates were low and that "it's difficult to explain why this is so".
Tegnell and Stefan Lofven, the prime minister, have long defended the soft lockdown, stating the disease is not something that can be defeated with a short-term fix but that people must learn to live with the virus.
A minute's silence was observed in Sweden's parliament to honour the 5000 people who died of the disease.