The majority of SARS-CoV-2 infections start to get better within several days, although infected people can shed virus and could infect others for anywhere from a few days to weeks. But some of COVID-19 infections have lasted for an extremely long time, in a persistent illness that is different from long COVID.
Researchers are still determining whether persistently long COVID-19 infections are likely to spawn dangerous variants of the virus, and the picture is complex. While people who are infected with SARS-CoV-2 can produce an astonishing number of viral particles - some estimates have suggested that as many as 30,000 viral particles can be generated in one infected person per day, for example - what is not known is how likely any one person is to transmit a variant made in their body to another person. There are also millions of infections, which does increase the likelihood that infectious variants will spread.
Transmission dynamics can depend on many things, and while it only takes a few viral particles to infect some people with SARS-CoV-2, it may take more of those pathogenic viral particles to infect others who have more robust natural immunity, for example.
Right now, scientists have said that there is "no consistent pattern" in how SARS-CoV-2 has evolved in chronic infections. Research has shown that viral evolution is a particular concern with immunocompromised patients, who can carry viruses with mutations that enable them to evade immunity.
One of the first patients recognized for having an extremely long case of COVID-19 was a UK man who was infected for at least 290 days. He had 42 positive PCR tests over that time, and though he was eventually cured with an antibody treatment, his wife had started to arrange a funeral multiple times. He had recovered from leukemia in 2019, and got sick with COVID-19 in 2020.
An American woman who was recovering from cancer, then infected with SARS-CoV-2, took 335 days to fully clear her viral infection. Her case of COVID-19 had gone through stages in which she would feel better, then get sick again. Researchers concluded that it was the same virus, however, based on a genetic analysis. There were many mutations in the viral samples taken from this patient too.
A man in the UK who had underlying conditions died from COVID-19 after a 505 day infection. He had been in and out of the hospital, taking 50 tests during that period.
"These were throat swab tests that were positive each time. The patient never had a negative test," Dr. Luke Blagdon Snell told the BBC. "And we can tell it was one continuous infection because the genetic signature of it; the information we got from sequencing the viral genome was unique and constant in that patient."
Researchers recently reported the case of another patient on the pre-print server medRxiv. This individual had a persistent COVID-19 infection that lasted for 471 days. There was evidence of repeated mutation in the viral samples they carried, and the rate of mutation in this individual was higher than the global average, the study authors noted. This patient was also recovering from cancer.
The study authors also stressed that chronic SARS-CoV-2 infections that go untreated can accelerate the evolution of the virus, and that this provides an opportunity for highly infectious variants to spread.
Sources: medRxiv, BBC, The Guardian, Nature Medicine, Science