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How is the UK variant more contagious?

Question: I’ve read that the new variant out of the UK is more contagious.  What does that mean in practice?  Does it just go from person to person faster or does an infected person pass it along to more people?

Answer: To my knowledge, we don’t know yet.  It could be that an infected person sheds more virus, making it easier to pass on to others.  It could be that the infectious period is different, making the timing of infection more speedy or making a person contagious for longer.  It could also be that the virus is better at binding to cells.  Lots of hypotheses.  So far, all the research we currently have shows that it does not result in more severe disease (yay!), but because it is more transmissible, we can expect it to cause more infections, which will cause more hospitalizations and deaths (uggghhh!).  Bulleted below is a summation of the pre-publication (not peer reviewed) findings that have been published in the last week, as well as a Public Health England update published a few days ago.  You can also find a bit more background in our Q&A of 12/21 and these syntheses from Contagion Live and WHO are both very informative. 

  • Public Health England Investigation of novel SARS-CoV-2variant: Technical Briefing 2 (published 12/28): This overview of the variant in the UK includes data on outcomes of a matched cohort study comparing outcomes of those hospitalized with the preexisting variant and new variant.  Findings show that 28-day case fatality rates were not statistically significantly different between the two groups, indicating that the variant does not cause more severe disease.  Researchers also examined whether reinfection was more likely with the variant, but found no statistically significant difference.  Finally, researchers used contact tracing data and found that a higher percentage of contacts of those with the variant themselves became infected (15.1% of new variant contacts vs. 9.8% of preexisting variant contacts).   
  • Estimated transmissibility and severity of novel SARS-CoV-2 Variant of Concern 202012/01 in England (posted on medRxiv on 12/26): Researchers use a transmission dynamic model to estimate that the variant is “56% more transmissible (95% credible interval across three regions 50-74%) than preexisting variants of SARS-CoV-2.”  This finding is consistent with other research indicating the variant is 70% more transmissible.  Researchers find no evidence that the new variant results in greater/lesser disease severity.  
  • S-variant SARS-CoV-2 is associated with significantly higher viral loads in samples tested by ThermoFisher TaqPath RT-QPCR (posted on medRxiv on 12/27): Researchers used a dataset of positive test results based on a test that can differentiate between the existing and new variant and found that viral load among those with the new variant was generally higher, which could have implications regarding transmissibility.  
  • SARS−CoV−2 Variant Under Investigation 202012/01 has more than twofold replicative advantage  (posted on medRxiv on 1/4): Researchers use data from GISAID to estimate the replicative advantage of the new variant compared with preexisting variants.  They find the replicative advantage is 2.24 [95% CI: 2.03−2.48], stating that “such significant replicative advantage and the fact that London serves as major international transportation hub suggest that the VUI−202012/01 strain will likely become globally dominant, hindering containment of the COVID−19 epidemics prior to massive vaccinations.”