News has emerged recently that the novel coronavirus has “mutated” to become more “aggressive”.

Researchers have found that the novel coronavirus falls into two different genetic types. The “L” type appears to have arisen by developing two genetic changes compared to the older “S” type. Despite being newer the L type appears more common, suggesting that it might be better at infecting and spreading among humans.

There is no need to be alarmed by these findings.

They come from 103 samples of the virus collected in different countries, at different times by different laboratories. This is a tiny sample of the virus that has caused COVID-19 in more than 100,000 people.

We don’t know whether this is a representative sample of the circulating viruses, so we can’t be certain that the L type is more common. We also have no information as yet on whether one type is more severe.


Where did the story come from?

Many news sources have covered the findings of this study by researchers from Peking University and other centres in China. For example, the Sun headline was that “Coronavirus has ‘mutated’ into TWO strains – with the most aggressive infecting 70% of patients, scientists claim”.


What is the basis for the claim?

The claim comes from research that analysed voluntarily submitted, publically available genetic data from 103 samples of the novel coronavirus collected from infected individuals, and also data from other coronaviruses.

The researchers found 149 genetic differences between the novel coronaviruses isolated from different people, most of which only occurred in one sample. The coronaviruses mostly fell into two types, based on the presence or absence of changes at two places in their genetic code. The researchers called these the “S” type (about 30% of the samples) and the “L” type (about 70% of the samples).

The S type of the virus appeared to be older but less common than the L type. This led researchers to suggest that the L type “might be more aggressive and spread more quickly”.

Almost all of the coronaviruses collected before early January (all of which were from Wuhan where the outbreak started) were of the L type (96%), but fewer of those collected after this (62%). The researchers suggest that this may be because the preventive measures have had more of an impact on the L type than the S type, but acknowledge that as yet the data is very patchy, so they would need more information before they can be sure of this.


What do trusted sources say?

The researchers themselves say that it is not clear whether the L type causes more severe disease (“is more virulent”) than the S type. As yet we have very limited information on the genetics of the circulating viruses, and how genetic variations might impact severity or infectiousness in humans, or in cells or animals in the laboratory.

As Dr Michael Skinner, Reader in Virology at Imperial College London has put it, “The authors reasonably construct some tentative hypotheses which can now usefully be tested as we gather more sequences elsewhere.” But cautions that “It is, however, too early to speculate on any practical consequences of the interesting observation”.

Analysis by EIU Healthcare


1.     Tang X et al. On the origin and continuing evolution of SARS-CoV-2. National Science Review, 2020; nwaa036 (Accessed 9 March 2020)

Reading list

1.     Science Media Centre. Expert reaction to study looking at whether there are two strains of the novel coronavirus. (Accessed 9 March 2020)