Boston Hospital
Photo: Kenneth Martin / ZUMA Wire / imago imagesSometimes a single event is enough to trigger a virological chain reaction in the corona pandemic. At a church service in Frankfurt, for example, more than a hundred people were infected; in a restaurant in East Frisia, there were almost two dozen.
Such so-called superspreader events are the nightmare of virologists. But how drastically the virus can spread when people from different countries and from different continents meet, experts now show in a previously published study.
To do this, they had examined how Sars-CoV-2 spread in the Boston area. According to the team led by Bronwyn MacInnis, an epidemiologist at the Broad Institute, a joint research facility between Harvard University and MIT (Massachusetts Institute of Technology), a conference held by a biotech company played a crucial role.
175 executives from the company met in a hotel in Boston at the end of February. At the time, the coronavirus didn't seem like a big issue in the US. But it was already rampant and spread unnoticed even in the stuffy conference rooms. From there it reached Singapore and Australia, and the researchers were able to trace the path of infection. Probably tens of thousands of corona cases go back to the meeting.
For the study, the researchers carried out analyzes of the virus genome of more than 750 people and then compared the genetic data. So they could understand the spread. When the virus multiplies in the body, the genome is passed on as well. Random mutations of genes are passed on - most of them have no influence on the properties of the virus. But the spread of such mutations allows conclusions to be drawn about the course of the epidemic.
The first corona case was discovered in Boston in late January. Up to 80 people probably brought the virus into the region by March. According to the virus' genetic fingerprint, some of the cases were due to outbreaks in Europe. A nursing home in Boston was also affected during the period, where 85 percent of patients and 37 percent of staff were infected. But apparently the virus has been contained - no other cases are known that can be traced back to this outbreak.
Double mutation at the conference
The meeting of biotech experts was completely different. The researchers suspect that the poorly ventilated rooms and close contact between people at the conference meant that the virus found ideal conditions to spread. The virus genome of 28 people was sequenced from the meeting for the study. All of them carried the same mutation called C2416T. The only known samples with this mutation prior to the meeting were from two people in France and were taken on February 29th.
The researchers do not know whether the virus was brought directly from France to the conference or whether it can be traced back to other cases in the greater Boston area. But they know that there was a second mutation at the meeting: G26233T. It must have arisen during virus replication in one of the participants. Anyone who got the virus from this person carried the double mutation.
Puzzling ways of infection
Jacob Lemieux, one of the study's co-authors and an infectious disease doctor at Massachusetts General Hospital, told the New York Times it was impossible to tell how many people contracted the virus in the months following the conference. More than 90 positive cases are known in the context of the conference. But according to the estimates, there are far more. In the greater Boston area alone, it could have been up to 20,000 people.
But even for this region there are puzzles with the routes of infection. For example, the researchers found the mutations in 51 samples in a homeless shelter. How the virus got there from the conference is unknown. The double mutation was later found in various US states, as well as in Europe, Asia and Australia. Lemieux estimates that tens of thousands of cases worldwide could be traced back to the conference.
The preprint study has not yet appeared in a scientific journal, but has only been published in advance by the researchers. According to Joshua Schiffer, a doctor who was not involved in the study and an expert on statistical models, it is a valuable contribution to better understanding the pathways of the virus. The conference was one of several events that contributed to the spread in the first few months after its occurrence in the United States, Schiffer told the New York Times.
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