A single index asymptomatic player who participated in the Major League Baseball matches in Philadelphia in end-June spread the virus to 20 other players of his team and to one employee of team B. Frequent testing helped identify players infected with virus days before they started showing symptoms. Among the 19 of 21 persons with COVID-19 who were symptomatic, symptoms developed an average of 2.3 days after collection of the test-positive sample.
The importance of testing asymptomatic persons for novel coronavirus and the import of mitigation strategies to decrease the spread of the virus have been highlighted in a super-spreading event involving team A that participated in the Major League Baseball matches in Philadelphia in end-June. A single index player spread the virus to 20 other players of his team and to one employee of team B.
The Major League Baseball developed new health and safety protocols before the July 24 start of the 2020 season based on CDC’s COVID-19 mitigation recommendations related to events and gatherings. These include non-pharmaceutical interventions such as mask wearing and social distancing, and frequent (every other day) testing for rapid identification of people infected with the virus even before they exhibited symptoms, isolation of infected people and quarantining of close contacts. In addition, the games were played without spectators. Results were published in the Morbidity and Mortality Weekly Report (MMWR).
Despite multiple mitigation strategies, including testing each player every alternate day being in place, the spread of the virus to other players could not be prevented. This was primarily because the virus spread during the asymptomatic phase of infection. To prevent further spread by asymptomatic players, the Major League Baseball initiated daily testing instead of alternate day testing for all players belonging to team A and B for the duration of the outbreak.
Frequent testing helped identify players infected with virus days before they started showing symptoms. “Among the 19 of 21 persons with COVID-19 who were symptomatic, symptoms developed an average of 2.3 days after collection of the test-positive sample,” says the team led by Dr. Meghan Murray from the Epidemic Intelligence Service, CDC which investigated the super-spreading event.
Single index case
The index case, who was still asymptomatic, received a positive test result on day four of the games before a three-game series with team B. The result was based on a sample collected on day two. The index case was immediately isolated and all players of team A and staff members who were in close contact with the index cases were tested.
On day five, after the second game with team B, three more players of team A tested positive. The following day, an additional eight team A players and staff member of team B tested positive. In the following days, eight more players of team A tested positive.
Genome sequencing revealed that 17 of the 18 samples that were sequenced had a single nucleotide variant, revealing that the super-spreading event was from a single index case.
Despite team A playing five games in all, spending a cumulative 11 hours on the field, no player from team B or team C, coaches or umpires became infected. According to the study, the virus spread had occurred outside the field. “Interactions outside of on-field play were likely the source of spread, and SARS-CoV-2 transmission risk between baseball teams while on the playing field appeared low,” the authors write. The employee of team B who was infected had apparently closely interacted indoors with team A tier 1 employees who subsequently tested positive.
“Social distancing measures have been effective in a variety of settings, and this outbreak provides additional support for their use during sporting events, such as baseball games,” the authors note. Universal mask-wearing policies provided additional protection for teammates. “Consistent adherence to these policies off-field also might have contributed to protecting the communities hosting the games,” they note.