If you have five or more symptoms in the first week of contracting Covid, you’re more likely to develop long Covid, according to Nature’s May 2021 study. It was the second-strongest predictor of long Covid (16.3 per cent). In the journal’s more recent study, those with more Covid-19 symptoms during primary infection were more likely to develop long Covid.

Likewise, symptoms of long Covid were between two and 6.5-fold more frequent in severe compared to mild Covid-19 cases (excluding those with loss of taste and smell), according to the same study.

This is to do with how the body responds to the virus. “If you’ve got a very active, quick-acting immune system, you knock it on the head quickly and you don’t get very many more symptoms […] whereas if you’ve got more symptoms, it’s gotten more into your body – that probably means it’s had an impact on your immune system in a different way,” says Dr Steves.

8. Viral load

The Cell article also found that a third of those presenting with long Covid symptoms had a higher level of coronavirus RNA in the blood early in the infection – also known as “viral load”.

This could explain why frontline workers are more likely to get long Covid, according to Prof Banerjee. “Long Covid seems to be more common in healthcare workers and frontline workers, which tends to suggest that people who are in the firing line for higher concentrations of virus are more likely to have long-term consequences,” he says. However, he admits that these workers are also more likely to come into contact with the virus – and thus more likely to catch Covid in the first place, which could explain the higher proportions of long Covid.  

9. Autoantibodies

The most influential factor in developing long Covid is the appearance of autoantibodies, according to the recent study in Cell. These autoantibodies are antibodies that mistakenly attack tissues in the body as they do, for instance, in auto-immune conditions like rheumatoid arthritis and lupus.

Autoantibodies were present in two-thirds of the cases of long Covid, according to Dr Heath.

However, vaccines significantly reduce the risk of long Covid

Being fully vaccinated could reduce your risk of long Covid, according to ONS data released this week. In a sample of UK adults aged 18 to 69 years, receiving two doses of a vaccine at least two weeks before contracting the virus was associated with a 41.1 percent decrease in symptoms 12 weeks later.

Likewise, a previous study at The Lancet found that two vaccine doses approximately halved the odds of having symptoms for 28 days or more after infection. “Vaccinations reduce the risk of long Covid,” adds Dr Steves.

It’s not entirely clear why but vaccines make the disease “less severe to start with”, Dr Steves says. “It’s also potentially primed the immune system to get on with its job”.

However, a vaccine won’t completely protect you from long Covid. “It reduces the risk but it doesn’t annihilate it,” she says.