Viruses mutate all the time, including the novel coronavirus that’s caused the global pandemic. But a variant that emerged in southeast England in September is causing particular concern, leading to an emergency lockdown in London over Christmas and causing countries including Canada, France and Germany to halt flights and suspend rail links.

1. Why is this mutation alarming?

Dubbed the “B.1.1.7 lineage,” the strain is defined by a set of 23 mutations, 14 amino acid changes and 3 deletions compared with the virus that emerged from Wuhan, China, in late 2019. That’s way more genetic change than typically observed. These include changes in key areas of the virus involved in its ability to infect cells. Preliminary analysis in the U.K. suggests it may be as much as 70% more transmissible than other circulating SARS-CoV-2 strains and may be contributing to a spike in cases in the country. Maria van Kerkhove, the World Health Organization’s technical lead on Covid-19, told the BBC on Dec. 20 that the WHO is working to understand the extent to which the virus may spread more easily, along with other human behavioral factors that may be driving transmission. It’s also looking at whether the mutation causes more severe illness and can evade the antibodies generated by vaccination.

2. What’s known about when it emerged?

The two earliest known specimens were collected in late September in London and the nearby area of Kent, and cases continued to be found through early December. One reason the new variant proliferated was that its emergence coincided with the festive season traditionally associated with increased family and social mixing. Scientists have hypothesized that the strain may have resulted from the transmission of the virus from a chronically infected patient.

3. How quickly has it spread?

As of Dec. 15, 1,623 virus samples were identified as belonging to the B.1.1.7 lineage. Of these, 519 were collected from patients in Greater London, 555 in Kent, and 545 in other regions of the U.K. including both Scotland and Wales. Cases have also been identified in Denmark, the Netherlands, Belgium and Australia. Since only about 10% of SARS-CoV-2 samples from U.K. patients undergo genetic sequencing, the number of known cases is an underestimate of the true number of infections.

4. Is it driving Covid-19 transmission?

Yes, but it’s hard to quantify because other factors, including the colder weather that’s encouraging people to congregate indoors, may also be contributing to an increase in Covid-19 cases. The new variant has become the dominant strain in some areas, suggesting it has a selective advantage over other variants, the U.K.’s New and Emerging Respiratory Virus Threats Advisory Group said in minutes from a Dec. 18 teleconference. The group noted that the B.1.1.7 lineage may result in an increase in the basic reproduction number, or the number of additional people likely to be infected by an individual with the virus, in the range of 0.39 to 0.93. That “demonstrates a substantial increase in transmissibility compared to other variants,” according to the group, which is chaired by Peter Horby, professor of emerging infectious diseases and global health at the University of Oxford. Infection rates in areas where the strain has been circulating have increased faster than expected, Public Health England said on Dec. 20, adding that there is no evidence the new variant is more likely to cause severe disease or mortality. Paul Hunter, a professor of medicine at the University of East Anglia’s Norwich School of Medicine, told the Science Media Centre that the new variant was responsible for 62% of SARS-CoV-2 infections in London in the week ending Dec. 9, up from 28% of infections there in early November.

5. How many other mutations are there?

Viruses constantly change through mutations that arise naturally as they replicate and circulate in their hosts. As a result of this ongoing process, many thousands of mutations and distinct lineages have already arisen in the SARS-CoV-2 genome since the virus emerged in late 2019. As mutations continue to arise, novel variants are increasingly found, with the vast majority of mutations having no apparent effect on the virus. However, scientists pay most attention to mutations like those that have occurred in the new U.K. strain involving the gene that encodes the SARS-CoV-2 spike protein, which is associated with viral entry into cells and influences immunity and vaccine efficacy.

6. Will vaccines still be effective?

The extent to which SARS-CoV-2 may evolve to escape immunity induced by infection or vaccination isn’t currently known. Studies are under way to understand that better. The WHO’s van Kerkhove told the BBC that current information suggests that the new variant doesn’t have any impact on the vaccines being rolled out. Over time, as more mutations occur, vaccines may need to be altered. This happens with seasonal flu, which mutates every year, and the vaccine is adjusted accordingly. Unlike flu, coronaviruses have a proof-reading mechanism that means they don’t mutate as quickly as influenza. Covid-19 vaccines that have so far proved effective in trials can be easily tweaked if necessary, the BMJ medical journal reported on Dec. 16.

(Adds information in answer to question 4.)

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