Eleven hours on the field with COVID, and nobody caught it

Before the 2020 season, Major League Baseball built a COVID-19 protocol and
played without spectators. Then, before a three-game series between teams A
and B, a team A player tested positive and was isolated.

Over the next eleven days, 19 more team A players and staff and **one team
B staff member** were diagnosed. The question the outbreak answers is the one
every sport was asking: does it spread on the field?

The exposure

Some team A players and coaches were asymptomatic and infectious while
playing. The report measures exactly how much:

**No on-field team B or team C player or staff member subsequently received a
clinical diagnosis of COVID-19.**

**The single team B case was a tier 2 employee whose official duties required
indoor interaction with team A.** Not a player. Indoors.

None of the 56 non-MLB staff who were tested — hotel staff and others with
possible contact — were positive.

The genomes

**Of 20 residual samples, 18 were suitable for whole genome sequencing. 17 had
identical consensus sequences and one differed by a single nucleotide.**

**These data were consistent with a single introduction resulting in a
super-spreading event.**

Not twenty separate infections from the community — one virus, one entry,
spread within the team. Phylogenetic comparison placed the genomes in **a
large clade dominated by sequences from the south-eastern United States.**

What actually did it

**The lack of evidence for on-field transmission, as demonstrated by the
absence of infections among opposing on-field team players and staff
members, pointed to indoor exposures as the likely means of SARS-CoV-2
spread.**

The protocol revisions on day 16 name what they thought the gaps were:

• Cloth face masks at all times except on the field of play
• Travel limited to essential staff
• No visits to gatherings of large groups — bars, lounges

Every one is about time spent off the field. The eleven hours of play were not
the problem; the bus, the clubhouse and the evening were.

What made it manageable

Daily testing once the outbreak began, with **results available on average
within 24 hours** (range 12–48). **People with COVID-19 were isolated at a
separate location from those quarantined.** Among the 19 symptomatic cases,
**symptoms appeared on average 2.3 days after the positive sample was
collected** (range 0–7) — meaning testing was finding people before they felt
anything, which is the only way to isolate them in time.

Games were postponed on days 8–14. No additional cases after day 11. Team A
resumed play on day 15.

Source: Centers for Disease Control and Prevention, MMWR.