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B.1.1.7 Transmisibility

Why is B.1.1.7 More Transmisible?
    
    Earlier this year, a new coronavirus strain named B.1.1.7 emerged in the United Kingdom and has now made its way across the globe. With B.1.1.7 came concerns about the potentially faster spread of the virus. If the new variant does spread faster, this would mean an increased number of COVID-19 cases and a greater need for an effective vaccine. Multiple studies have found that this new strain is indeed more transmissible than previous strains of SARS-CoV-2. (See A New Coronavirus Strain for more information about B.1.1.7) A recently published article in preprint at the Harvard T.H. Chan School of Public Health, investigates the reason behind the high transmissibility of the B.1.1.7 variant. In this study, researchers specifically looked into the viral load of infected patients as well as how long the virus lasted in each individual. The details of the study are summarized below.

About the Study

    The study, conducted at the Harvard T.H. Chan School of Public Health, enrolled a cohort of 65 individuals who had tested positive for COVID-19.  Of these 65 participants, 7 were infected with the B.1.1.7 variant. In every patient, the researchers measured 3 different variables: time from first detectable virus to peak viral concentration (proliferation time), the time from peak viral concentration to the return of initial lowest viral concentration (clearance time), and the highest viral concentration for each individual. These measurements help determine whether and how viral load or infection time differs between the B.1.1.7 variant and the non-B.1.1.7 variant. The results drawn can then help explain why B.1.1.7 has a higher transmissibility.

Study Results

    For individuals infected with the non-B.1.1.7 variant, the average proliferation phase was 2 days and the average clearance phase 6.2 days; this amounts to an overall infection period of 8.2 days. In contrast, for individuals infected with B.1.1.7, the average proliferation phase was 5.3 days and the average clearance phase was 8 days. Thus, the overall duration of COVID-19 infection lasted 13.3 days, a significantly longer period than the non-B.1.17 group. (See below- blue represents non-B.1.1.7 and red represents B.1.1.7)

   
    When measuring the viral load in participants, researchers found that there was no significant difference between patients with and without the B.1.1.7 variant. The highest viral concentration in patients without the B.1.17. variant was 8.2 log10 RNA copies/ml (20.2 Ct) while the peak concentration in patients with the B.1.1.7 was 8.5 log10 RNA copies/ml (19 Ct). (See below)


Conclusion

    To summarize, researchers found that patients with B.1.1.7 had longer infection periods than patients with a non-B.1.1.7 variant. There was also no significant difference in viral load between the two groups, indicating that viral load does not greatly impact transmissibility. These results mean that longer infections are likely the factor that's fueling the high transmissibility of B.1.1.7. Patients with the B.1.1.7 variant are infected (5.3 days) longer and therefore have a greater chance of spreading the virus to others compared to those infected with a non-B.1.1.7 variant. Going forward, individuals who are infected with the B.1.1.7 variant should have longer quarantine periods to reduce the spread of the virus.

Sources

1. Kissler, Stephen, Joseph R. Fauver, Christina Mack, Caroline G. Tai, Mallery I. Breban, et al. "Densely sampled viral trajectories suggest longer duration of acute infection with B.1.1.7 variant relative to non-B.1.1.7 SARS-CoV-2." Preprint, 2021.

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