The coronavirus, as seen in an illustration from US officials, developed a more aggressive strain after crossing to humans, researchers believe

Scientists have identified two strains of the novel coronavirus causing infections, one more aggressive than the other, indicating that the disease has mutated at least once.

In a new study published as Britain was told to expect a pandemic, Chinese researchers suggest that after Covid-19 crossed into humans, the original strain evolved into a new type and both of these are now circulating.

Although further research is needed, the preliminary study, carried out at Peking University’s school of life sciences and the Institut Pasteur of Shanghai, sheds some light on how the disease is evolving.

The researchers identified two strains, with the more aggressive accounting for 70 per cent of those they analysed and a less aggressive strain making up the rest.

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