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The emergence of the epidemic in Louisiana

The first case of COVID-19 was identified in December 2019 in Wuhan, China. By February 2020, the epidemic had spread and was detected in Europe, the Middle East, and in numerous other regions throughout the world. In the United States, the first case was identified retrospectively as having occurred a little earlier, in January 2020. It would seem that large gatherings of people accelerated the transmission of the virus and helped the epidemic to grow. For example, more than a million people joined in the Mardi Gras parade in New Orleans on the 25th of February 2020. This event drew the attention of American researchers and they studied it in order to understand how such a large-scale event could have contributed to the massive spread of SARS-CoV-2.

The researchers sequenced the SARS-CoV-2 virus genomes in cases from New Orleans and other places in Louisiana at the beginning of the epidemic, during the first wave between March and May 2020. They compared these genomes to those in other states. They were thus able to reconstruct the dates and the origins of the appearance of the virus in Louisiana. In addition to their genome data, the researchers studied the epistemological data and the movement of people in order to better understand the emergence of the virus in this state.

Their first conclusion was that the emergence of the virus in Louisiana had its provenance in another American state rather than abroad. The genome of the 1st case of COVID-19 detected in Louisiana, the 9th of March 2020, was very close to other genomes in patients infected in other parts of the country. In addition, the SARS-CoV-2 found in Louisiana showed very little genetic diversity compared with other American states. The authors of the study compared this diversity to that of an epidemic on a cruise ship: the origin of the virus that spread in Louisiana may have come from a single source of contamination. This single source origin was responsible for the majority of virus transmission during the first wave of the epidemic.

According to phylodynamic analyses carried out by the researchers, the virus may, however, have emerged and spread some weeks before the parade. However, this gathering was undoubtedly responsible for an increase in transmission of the virus in New Orleans, with hundreds of cases of SARS-CoV-2 infection just after the event: it was what we call a super-spreader event. The acceleration in transmission was directly imputable to this event, unlike later waves of the epidemic that were due to new, more infectious, variants. Finally, the scientists were able to show that the cluster that occurred during Mardi Gras in New Orleans was at the origin of the spread of the virus to other states: the majority of cases of COVID-19 in Mississippi and Alabama came from New Orleans.

The emergence of COVID-19 in Louisiana could therefore have been due to a single infected person. The Mardi Gras gathering then accelerated the spread of the virus across the state and into neighbouring states. In general, mass gatherings at the beginning of 2020 did contribute significantly to increased transmission rates of the virus.

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