Last month, two European patients were confirmed to have been reinfected with coronavirus, raising questions over how long immunity to the virus lasts. 

The cases, in Belgium and the Netherlands, came a day after researchers in Hong Kong revealed that a 33-year-old man was reinfected with a different strain of the virus four and a half months after being declared recovered – the first such re-infection to be determined through genetic sequencing.

Genetic analysis revealed that his second infection, which he caught on a trip to Europe, was caused by a different strain of the virus.

All viruses mutate over time. Analysing the DNA of pathogens can spot any changes, which can make them weaker or deadlier.

Doctors warned their findings prove “re-infection can occur just a few months after recovery”. They said it was likely that immunity is short-lived and that antibodies against Covid-19 fade quickly.

But the unidentified man did not have symptoms of Covid-19 the second time and it was only discovered through screening at an airport. This may suggest he had some level of immunity that protected him from severe disease, experts said.

The cases have fuelled fears about the effectiveness of potential vaccines against the virus. Previous antibody studies have shown that immunity wanes over time, raising questions about the effectiveness of any future vaccine. 

The team behind the Oxford University vaccine – touted as the front-runner – have already said any immunisation may have to be given annually, like the flu jab.

At the same hearing, Mr Hancock urged Britons to change their attitudes to sickness, saying they would often go into work when sickly.

“The British approach to turning up at work if you are soldiering on if you’ve got a runny nose is an outlier internationally, and people should actually protect and look after their colleagues,” he said, urging those with symptoms which could mean they had Covid-19 to remain at home. 

Most testing of immunity has focused on antibodies. Antibody testing has been dogged by problems, as tests have proved unreliable and it is not clear how much immunity antibodies confer. 

But other types of substance can produce immunity, including white blood cells called T-cells, which are increasingly thought to play a key role in fighting the virus. Scientists believe the part played by T-cells may explain why some groups, in particular children, appear to be more immune to coronavirus.

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