Biogen's superspreader Boston event in February may have led to 245,000 Covid-19 cases in US, other nations: Study
While Massachusetts accounted for most early spread related to the conference, Florida accounted for the greatest proportion of cases overall
A conference in Boston earlier this year, which has already been flagged as a coronavirus super-spreading event, may have infected about 245,000 Americans as well as people from other countries, suggest researchers. They emphasize that while these estimates are provisional, they “convey the likely scope” of regional, national and international spread resulting from a single super-spreading event early in the Covid-19 pandemic.
Sustained local transmission of coronavirus in the Boston area was first detected in early March 2020, and with it, case clusters began to appear. The first large cluster was recognized in the context of the international business conference by Biogen held in Boston from February 26-27. Public health investigation with contact tracing identified approximately 100 cases associated with this conference, raising suspicion that a super-spreading event had occurred there. The authors sequenced SARS-CoV-2 (the virus that causes Covid-19) genomes from 28 of these cases, which reveals “the signature of super-spreading".
The analysis determines that one strain of the coronavirus, named C2416T, was the culprit. According to the authors, a single person carried this particular version of the virus to Boston and it spread via the conference. They note that a “single introduction had an outsize effect on subsequent transmission” because it was intensified by super-spreading in a highly mobile population very early in the outbreak before many public health precautions were put in place. Another called G26233T ended up in 88,000 of these cases.
The scientists argue that the C2416T/G26233T sublineage, which arose in the context of the conference, was exported from Boston to at least 18 US states as well as to other countries including Australia, Sweden and Slovakia, with evidence of community spread in many places. While Massachusetts accounted for the most early spread related to the conference, Florida accounted for the greatest proportion of cases overall (29.2%).
"Genome data reveal that the impact of the conference was far larger than the approximately 100 cases directly associated with the event. Using state-reported case counts, we estimate that approximately 50,000 diagnosed cases (44,000-56,000) in the US resulted from conference-associated viruses; of these, 46% (40.4-51.8%) were in Massachusetts. Through November 1, 2020, we estimate that a total of 245,000 (205,000-300,000) cases marked by C2416T, and 88,000 (56,000-139,000) cases marked by G26233T, were linked to the conference in the US," explain investigators in the study published in the journal Science.
The study was undertaken to understand the role of super-spreading events in regional and national transmission, which experts explain is "critical for prioritizing public health interventions". “Massive ongoing transmission globally underscores that most countries have not found effective ways to control the spread of the virus; better understanding of transmission dynamics could contribute to more targeted and effective responses to the pandemic,” they add.
The analysis shows that viruses containing C2416T could be found in 29 states. “C2416T began to appear in multiple other US states in early March and increased rapidly in frequency. The effect of this spread was long-lasting. By November 1, 2020, viruses containing C2416T could be found in 29 states, and this lineage contributed 1.9% (675/35,566) of all US SARS-CoV-2 genomes. States with the largest numbers of cases included ones with known travel by or reported epidemiological links to conference participants returning from the meeting, including Florida, (125/1552 genomes contain C2416T), North Carolina (20/94 genomes), and Indiana (10/42 genomes),” explains the team.
The C2416T mutation may have originated outside the US. The researchers checked a global database of samples of the coronavirus and found it in two French patients diagnosed on February 29. In the US, it was only found in patients associated with the conference before March. “Taken together, this strongly suggests low-level community transmission of C2416T in Europe in February 2020 before the allele came to Boston via a single introduction, which was then amplified by super-spreading at the conference,” say researchers.
The second variant, G26233T, with a strong conference association, emerged during or immediately after the conference, as it was first seen in 7 of 28 individuals with known conference exposure, including in one sample at an intermediate frequency (26%). “It is not seen elsewhere in any public genome databases prior to cases associated with the conference,” the report states.
An interesting finding is that within a month of the Biogen conference, the virus strain introduced there had made its way to Boston-area homeless shelters. Tests at the shelters, affiliated with the Boston Health Care for the Homeless Program, found 14 strains of the coronavirus, four of which appeared to have become superspreaders. The researchers found that two clusters of cases that resembled superspreader events were associated with the virus from the conference. “Two of the clusters descended from the conference-associated C2416T lineage, including one that contained C2416T/G26233T. Cases associated with the homeless shelters likely resulted from a mix of super-spreading events and more general transmission, although we lack the detailed epidemiological data to explore their history in depth,” explain scientists.
Stating that this study provides clear evidence that super-spreading events may ‘profoundly’ change the course of an epidemic, the team concludes that prevention, detection and mitigation of such events should be a priority for public health efforts.