For the second time in seven months, Boris Johnson is self-isolating in his Downing Street residence – this time, after coming into contact with somebody who might have Covid-19. On Thursday 12 November, the Prime Minister spent about 35 minutes with Tory MP Lee Anderson, who lost his sense of taste the next day, and later tested positive. Three days later, Mr Johnson was contacted by a clinician from NHS Test and Trace, and told to self-isolate.
As anybody who followed the news this spring will know, the Prime Minister has already had Covid. He tested positive for the virus on March 27, and was later taken to hospital, where he spent three days in intensive care.
But in a video on Monday, Mr Johnson said: “It doesn’t matter that I’ve had the disease… We’ve got to interrupt the spread of the disease and one of the ways we can do that now is by self-isolating for 14 days when contacted by Test and Trace.”
So how long does immunity last, and is it possible to catch the disease twice?
Can you catch Covid twice?
In short: yes, but it’s very unlikely. The first case of Covid reinfection was documented in August in Hong Kong, where a study from the University of Hong Kong’s Department of Microbiology said that an “apparently young and healthy patient had a second episode of Covid infection which was diagnosed 4.5 months after the first episode”. Tests showed that the strain of virus was genetically different from the first infection – proving that the positive test really was picking up a new infection, rather than just lingering particles from the first infection.
And last month, the journal Lancet Infectious Diseases reported that a man in the US state of Nevada tested positive for Covid-19 about a month after recovering from his first infection. Worryingly, the second hit appeared more serious: the otherwise healthy 25-year-old needed hospital treatment after his lungs could not get enough oxygen into his body. He later recovered.
But such incidents remain newsworthy precisely because they are so rare. In general, you are unlikely to be infected twice with the same infectious disease. So far, Sars-Cov-2 is behaving broadly like other infectious viruses, supporting the idea that herd immunity can be achieved once a vaccine is rolled out.
In the early weeks of the pandemic, for example, South Korea’s public health agency investigated 285 cases in which patients tested positive twice for Covid in short succession. Researchers were unable to grow live virus from any of the samples, and none of the infected people appeared to have spread the virus to others – suggesting that the positive tests were just picking up old virus from the original infection.
“I would say reinfection is possible, though not likely, and I’d think it would be rare,” Dr Angela Rasmussen, a virologist at Columbia University in New York, told the New York Times in the summer. “But even rare occurrences might seem alarmingly frequent when a huge number of people have been infected.”
How long do antibodies last?
Much of the discussion around Covid immunity has centred around antibodies, proteins in the blood created by the body to combat a specific antigen. In his video on Monday, Mr Johnson declared he was “bursting with antibodies”, seven months after recovering from the illness.
But the prime minister might not be correct. A study by Imperial College London, published last month and involving 365,000 people, showed that protective antibodies fell by more than a quarter in just three months. The study showed that by June, after the first wave of the pandemic, six per cent of the population had developed antibodies. Three months later, that figure had dropped to 4.4 per cent, with most of the decline happening in just six weeks.
Those findings were certainly disappointing, say scientists, but it does not mean that immunity fades away after just a few months. Whilst antibodies appear to fade quickly, they are not the only part of our immune system’s protective armour: there is still great debate among researchers about the role played by T-cells, white blood cells that target invaders. It is likely that both antibodies and T-cells are important, and that any vaccine should attempt to induce a durable response involving both. In trials, for example, the Oxford vaccine being developed in partnership with AstraZeneca stimulates T-cell and antibody response.
Indeed, in a study earlier this month, scientists from Public Health England and the University of Birmingham found that coronavirus patients retain T-cell immunity for at least six months after infection, even when antibodies become undetectable. The study, described as “reassuring” by experts, boosted hopes that vaccines will generate long-term immunity.
How long do symptoms last?
Because Covid is a relatively new illness, scientists are still unsure exactly how long symptoms last. According to the Government, if you contract Covid you have to self-isolate for 10 days, starting from when you first noticed symptoms or first received a positive result, whichever came later.
One detailed medical report of a waitress on the Diamond Princess cruise ship (a disease hotspot) who suffered a mild form of the disease showed that she displayed symptoms for 10 days. And a study of nine German patients who were also only mildly affected showed that they displayed symptoms for between eight and 11 days.
People with more severe forms of the disease will take longer to recover – a study of 138 patients who were hospitalised in China showed that some patients were in hospital for up to two weeks, although the average stay was 10 days.
A small number of people are left with what campaigners are calling “long Covid”; fatigue, psychiatric problems, and pain that lingers for weeks or months after the initial infection has passed.