Covid infection induces high levels of immunity but patients still able to carry virus, study shows

Infection with Covid-19 provides up to 83 per cent protection against the virus for at least five months, a new government study has shown - though experts have warned that those with natural immunity may still be able to carry and transmit the pathogen to others.

Research conducted by Public Health England (PHE) has found that prior infection offers levels of protection similar to those induced by the Covid-19 vaccine, with only a small proportion of people retesting positive after primary exposure to the virus.

In those rare instances of reinfection, most did not display any symptoms, according to the first report from PHE’s Siren study - which has been regularly testing tens of thousands of healthcare workers across the UK since June.

Between 18 June and 24 November, scientists identified 44 potential reinfections - two probable, and 42 possible - among 6,614 participants who had shown evidence of previous disease. None of these cases were tested during the first wave but all had Covid-19 antibodies.

Once adjustments were made for the incidence density rate per 100,000 people over this time frame, the scientists concluded that prior infection offered people up to 83 per cent protection, the effects of which last for a minimum of five months.

However, this means that many people who contracted the disease in the first wave may now be vulnerable to catching it again, PHE said, as it urged people who have been infected not to let their guard down.

PHE said although those with antibodies are unlikely to become ill again with Covid-19, early evidence from the next stage of the study suggests that some of these individuals can still carry high levels of virus and continue to pass it on to others.

The scientists were also unable to establish the durability of the body’s immune response. “Will it last 6 months, 1 year, 2 years? We're afraid we'll have to wait for time to answer those questions fully,” said Professor Susan Hopkins, a PHE senior medical advisor and the Siren study lead.

The study, which has not been peer-reviewed, will continue to follow participants for 12 months to explore how long immunity may last, the effectiveness of vaccine responses and to what extent people with protection are able to carry and transmit the virus.

The findings of the research do not apply to the new coronavirus variant which first emerged in Kent last autumn. Further laboratory analysis is underway to understand whether and to what extent antibodies also provide protection against B.1.1.7.

Despite the lingering question marks around long-term immunity, Prof Hopkins described the study as “good news”.

“Prior infection looks as good as [a] vaccine, at least at this time interval, which is very good news for the population,” she said. “It will help alongside the vaccines to give a level of immunity in the population that will start to reduce transmission.

“New infections can come, and you can definitely be reinfected after a primary infection, though the risk of severe disease is extremely low and the vast majority of people have a very good level of protection.

"It is good that it's protecting people, but it's not 100 per cent protective, and therefore people still need to follow the rules, until we know more about this, on the durability of the response, and also understand better why some of these individuals have not responded, or is this particular to a certain group."

Prof Hopkins explained that in those cases of reinfection, the virus was detected in the back of patients’ throats and noses, and likely to have been “rapidly cleared” by the immune system before being given the chance to generate any symptoms and disease.

“Even if you're infectious, you're likely to be infectious for a very short of period of time but we have clearly found people who are infectious and people need to have a note of caution that we don’t know who these people are,” said Prof Hopkins.

“We can’t predict it on your age, your sex or the part of the country you live. We therefore need to take precaution still. It’s really important now especially with such a high amount of virus circulating.”

Prof Hopkins said that when the study was designed, researchers were looking to ensure they could capture the minimum immunity that would be acceptable for a vaccine.

She continued: "And at that time, the vaccine effectiveness studies were setting out to look at 'did vaccines have 50 per cent to 60 per cent efficacy in reducing future symptomatic infection?'

"And if you look at this from a symptomatic point of view, we're seeing a very small proportion - 13 out of the 44 - have had symptomatic infection."

Professor Eleanor Riley said the research was “good news in terms of the long term trends of the pandemic.

“However, asymptomatic reinfections are not zero, so you cannot assume that just because you have had the virus before that you can’t be infectious.

“These data reinforce the message that, for the time being, everyone should consider themselves to be a potential source of infection for others and should behave accordingly.”