Convention studies detail Omicron's early moves in US

The pre-Thanksgiving spread of the Omicron COVID-19 variant among fully vaccinated attendees of a New York City Anime convention was an early sign the variant was able to evade vaccines, according to a study today in Morbidity and Mortality Weekly Report (MMWR).

But despite a high attack rate among close contacts, the convention itself was likely not a super-spreading event, according to another study published today in MMWR, and it had limited impact on the variant's introduction to the United States, researchers said.

The convention, held at the Javits Center from Nov 19 through the 21, 2021, included 53,000 attendees from 52 US jurisdictions and 30 foreign countries, the authors said. Masks were required during the convention, and attendees had to show proof of at least one dose of COVID-19 vaccine.

High spread in close contacts

The Omicron variant was detected in patient A, an attendee from Minnesota who was fully vaccinated and boosted but developed symptoms of COVID-19 the day after attending the convention.

On Dec 2, 2021, the Minnesota Department of Health identified patient A as he first case of community-acquired COVID-19 in the United States caused by the Omicron variant. Twenty-nine close contacts from the convention were identified, and 23 were interviewed. The close contacts socialized at bars and restaurants without masks on, and performed karaoke together.

All 23 close contacts were fully vaccinated, including 11 (48%) who had received a booster dose; all 23 sought testing, and 16 (70%) received a positive SARS-CoV-2 test result. Subsequently 6 household members of the positive 16 cases also tested positive for the virus in the days following the convention. 

Among the 11 attendees who had received a COVID-19 booster dose before the convention, 6 (55%) received a positive test result. Among 12 attendees who had not received a booster, 10 (83%) received a positive test result. None of the case-patients were hospitalized or died, and all reported mild, self-resolving symptoms, most notably nasal congestion.

"The high attack rates among fully vaccinated persons illustrate Omicron variant's partial escape from vaccine-induced immunity," the authors wrote. "Potential contributing factors to the high attack rates include unmasked and prolonged contact in social settings and residential accommodations. All Omicron cases identified from the larger investigation were associated with this group of close contact attendees."

Mitigation strategist likely helped

In the second study on the Anime convention, investigators found the convention itself was not a super-spreading event, and Omicron case numbers were low.

Similar to what was seen in New York City at the time of the convention, the Delta variant accounted for 75% of all COVID-19 cases identified at the convention. Omicron was detected in only 25% of case-patients who underwent genomic sequencing.

Investigators tracked 34,541 attendees with available contact information and gathered surveillance data, which identified test results for 4,560, including 119 (2.6%) who tested positive for COVID-19. One attendee was hospitalized due to the virus, and none died.

"Compared with test-negative respondents, test-positive respondents were more likely to report attending bars, karaoke, or nightclubs, and eating or drinking indoors near others for at least 15 minutes," the authors wrote. "Among 4,560 attendees who received testing, evidence of widespread transmission during the event was not identified."

The Javits Center was equipped with HEPA filters, and attendees were required to have documented receipt of at least 1 dose of COVID-19 vaccine and to use face masks while indoors, the authors said. These factors limited the spread of COVID-19, they noted.

"The 7-day average percentage of positive test results in NYC on December 5, 2021, (3.0%) was similar to that in this investigation (2.6%)," the authors said. "Omicron variant accounted for <5% of sequenced cases in NYC by December 4, 2021; transmission could have been higher had the convention occurred after Omicron became the dominant variant."

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