FACT CHECK: Is the COVID-19 virus becoming less deadly and contagious over time?
Updated: Jun 17
NOT SUPPORTED BY EVIDENCE. As of June 12, most scientists agree that new mutations are not causing the COVID-19 virus to become less deadly or less transmissible.
In the last two weeks, some doctors in Italy have started to notice something strange. The number of deaths, cases, severely ill patients, and even the viral load among patients—which refers to the number of viral particles carried by an infected person—have been dropping or staying the same, according to some early experiments. The phenomenon led Alberto Zangrillo, an Italian doctor, to tell reporters that he believes the virus is weakening and may disappear altogether without a vaccine.
He told Reuters that, based on his observations at the San Raffaele Hospital in Milan, the average viral load is “absolutely infinitesimal” compared to one or two months ago.
However, the broad consensus among the scientific community is that there is no evidence the virus is getting weaker. The changes observed by Zangrillo and other Italian doctors may instead be the result of other factors, such as reduced exposure to the virus and more widespread testing.
No evidence supports the idea that mutations affect transmission
There is a certain logic to the idea that a virus might get weaker over time. Viruses naturally mutate—that’s why you need to get a new flu vaccine each year—and there’s a trade-off between how contagious and how deadly a virus is. If a virus kills or incapacitates the host too quickly, it might prevent them from moving around and spreading the virus to more people. Over time, the argument goes, milder versions of the virus may spread farther and could eventually win out over more deadly versions.
While that can happen, research doesn’t support the idea that COVID-19 is going through a similar process. Coronaviruses mutate at a slower pace than a flu virus, and researchers at Johns Hopkins University say the mutations so far have not resulted in different strains of the virus. Genomic studies have yet to identify a mutation that makes any meaningful difference to transmission.
Maria Van Kerkhove, a doctor and spokesperson for the World Health Organization (WHO), said in a press conference that the virus was not becoming milder or less contagious.
“The majority of people have a more mild infection, some have a moderate infection with pneumonia, and then about 20 percent of individuals will have a severe disease. That is consistent,” she said. “So in terms of the transmissibility, that has not changed. In terms of the severity, that has not changed.”
Reuters later contacted experts at six universities and research centres in the US and UK who agreed with the WHO’s conclusions that a change in the virus is not supported by the scientific literature they are familiar with.
There are other possible explanations
It may be true that, in Italy, the virus is mutating and that doctors are seeing fewer deaths and a lower viral load, but that does not prove that the mutation is affecting transmission or the deadliness of the virus. Instead, there are other factors that could explain what Italian doctors are seeing.
Van Kerkhove and Michael Ryan, another doctor at the WHO, attributed the changes to human intervention. “It may not be that the virus itself is becoming less potent. It may be that we are, as a community and as a globe, successfully reducing the number, intensity, and frequency of exposure to that virus,” Ryan said in the same press conference.
Measures like widespread testing, isolating new cases, and contact tracing could be reducing the duration and intensity of exposure to COVID-19, which can have an effect on how severe the illness becomes.
Another theory, suggested by Martin Hibberd at the London School of Hygiene & Tropical Medicine, is that doctors are now observing more patients with mild cases in countries where the outbreak is under control—as opposed to focusing only on urgent cases—which would give the impression that the virus is changing.
The exact reason why the virus has become less severe in Italian hospitals is difficult to pin down. What is clear, though, is that there is little scientific support for the idea that there is a mutation causing the COVID-19 virus to become less contagious or less lethal over time.