The new Covid December 2020 mutation in UK is believed to be 70% more contagious than the original version. Could one reason be that Airborne transmission by normal breathing is now a dominant form of transmission? If this is the case, then are standard masks insufficient for any indoor setting, and should face masks that have filtration capability of N95 (or even stronger) be made mandatory?

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The higher transmissibility of this VUI-202012/01 strain (if it is confirmed) is almost certainly due to mutations in the spike gene of the virus; see preliminary report from COG UK; another version of that with slightly more analysis confirms that one of the (17) mutations involved had been previously confirmed as causing higher transmissibility in a mouse model

N501Y has been associated with increased infectivity and virulence in a mouse model (Gu et al. 2020).

I think even the epidemiological implications haven't been quite dissected yet; I've seen the newspaper reports of (up to) 70% more contagiousness and R increase by 0.4 and that it's responsible for more infections in some areas, but I haven't seen any more scientific publications on the epidemiology of that strain (unlike for its genetics).

It's also premature to conclude anything about masks with respect to transmission of this strain.

N.B., the ECDC has a lengthier report on it now, although it doesn't add much in the way of facts (other than the strains was seen in a few cases in continental Europe too.)

Science has a more insightful coverage, in that some experts have questioned the epidemiological conclusions as possibly rushed, but also there being a real concern because N501Y has apparently been seen in a different strain in South Africa that apparently also spreads faster, although data on this is still/also unpublished:

A fortunate coincidence helped show that B.1.1.7 (also called VUI-202012/01, for the first “variant under investigation” in December 2020), appears to be spreading faster than other variants in the United Kingdom. One of the PCR tests used widely in the country, called TaqPath, normally detects pieces of three genes. But viruses with 69-70del lead to a negative signal for the gene encoding the spike gene; instead only two genes show up. That means PCR tests, which the U.K. conducts by the hundreds of thousands daily and which are far quicker and cheaper than sequencing the entire virus, can help keep track of B.1.1.7.

In a press conference on Saturday, chief science advisor Patrick Vallance said that B.1.1.7, which first appeared in a virus isolated on 20 September, accounted for about 26% of cases in mid-November. “By the week commencing the 9th of December, these figures were much higher,” he said. “So, in London, over 60% of all the cases were the new variant.” Boris Johnson added that the slew of mutations may have increased the virus’s transmissibility by 70%.

Christian Drosten, a virologist at Charité University Hospital in Berlin, says that was premature. “There are too many unknowns to say something like that,” he says. For one thing, the rapid spread of B.1.1.7 might be down to chance. Scientists previously worried that a variant that spread rapidly from Spain to the rest of Europe—confusingly called B.1.177—might be more transmissible, but today they think it is not; it just happened to be carried all over Europe by travelers who spent their holidays in Spain. Something similar might be happening with B.1.1.7, says Angela Rasmussen, a virologist at Georgetown University. Drosten notes that the new mutant also carries a deletion in another viral gene, ORF8, that previous studies suggest might reduce the virus’s ability to spread.

But further reason for concern comes from South Africa, where scientists have sequenced genomes in three provinces where cases are soaring: Eastern Cape, Western Cape, and KwaZulu Natal. They identified a lineage separate from the U.K. variant that also has the N501Y mutation in the spike gene. “We found that this lineage seems to be spreading much faster,” says Tulio De Oliveira, a virologist at the University of KwaZulu Natal whose work first alerted U.K. scientists to the importance of N501Y. (A preprint of their results on the strain, which they are calling 501Y.V2, will be released on Monday, De Oliveira says.)

[...] In the case of N501Y, more young people may be getting sick because many more are getting infected; Oliveira says some recent post-exam celebrations in South Africa have turned into superspreading events. Studies in cell culture and animal experiments will have to show how a virus with several or all of the mutations carried by the new variant compares with previous variants, says Drosten.

[...] In the lab, Gupta’s group found that virus carrying the two mutations [ 69-70del and D796H] was less susceptible to convalescent plasma from several donors than the wildtype virus. That suggests it can evade antibodies targeting the wildtype virus, Gupta wrote in a preprint published this month. He also engineered a lentivirus to express mutated versions of the spike protein and found that the deletion alone made that virus twice as infectious. He is now conducting similar experiments with viruses that carry both the deletion and the N501Y mutation. The first results should appear just after Christmas, Gupta says.