A recent report published by the Centers for Disease Control and Prevention (CDC), USA, has demonstrated one of the earliest outbreaks of omicron infection in the US. The report reveals that the chances of omicron transmission among people attending a large-scale indoor convention are high. However, people who have received coronavirus disease 2019 (COVID-19) booster vaccination are less likely to get the disease from infected participants. The detailed findings are available on the CDC’s Morbidity and Mortality Weekly Report.
Outbreak of omicron infection in New York convention
The report describes one of the earliest outbreaks of omicron variant of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the United States. A Minnesota resident attending a large indoor convention with 53,000 participants in New York was detected with SARS-CoV-2 infection on November 23, 2021. The genome sequencing analysis revealed that the person (index case) was infected by the omicron variant. On December 2, 2021, the Minnesota Department of Health informed the CDC about the case.
The New York convention was conducted with 53,000 participants from 52 US jurisdictions and 30 foreign countries during November 19 – 21, 2021. According to the convention rule, all participants must have at least one vaccine dose and wear a mask indoors. The index person with omicron infection was in close contact with 29 fellow participants for five days. In addition to attending several convention sessions, the index person was engaged in many informal activities, including unmasked visits to social gatherings.
The index person started developing symptoms on November 22 and immediately informed all 29 close contacts about the incident. All contacts self-isolated themselves and got tested over the next few days. A separate investigation was also conducted by the respective state and local health departments to determine SARS-CoV-2 transmission among all participants and their household members.
The demographic, epidemiologic, and clinical information was collected from the index person, close contacts, and household contacts. All information was subsequently reviewed by CDC, US.
Among 30 participants, including the index person and all 29 close contacts, 23 were interviewed. All interviewees were fully vaccinated, with 11 reported having booster vaccination.
Of 23 interviewed participants, 16 tested positive for SARS-CoV-2. The sequencing of samples collected from five participants identified omicron variant sub-lineage BA.1. The same omicron sequence was identified in three family members of a single participant.
Among 11 interviewed participants with booster vaccination, six tested positive for SARS-CoV-2. The infection rate was higher among participants who did not receive a booster dose. Specifically, among 12 participants without booster vaccination, ten tested positive for SARS-CoV-2. The most commonly reported symptoms were nasal congestion, fatigue, cough, and sore throat, and the average duration of symptoms was 11 days. No hospitalization or death was observed among SARS-CoV-2-positive participants.
During the convention, none of the participants opted for international travel. After returning from the convention, 16 participants with a positive test result came in close contact with a total of 20 household members. Of household contacts, 19 were fully vaccinated, including 10 with booster vaccination.
Among 18 household members who got tested, six had received a positive result, including four with booster vaccination received more than 14 days before exposure. The most commonly reported symptoms were nasal congestion, fatigue, cough, runny nose, and altered taste, and the average duration of symptoms was 13 days. None of them were hospitalized or died.
Outside the cluster of index person and close contacts, only limited evidence of viral transmission was identified in the convention.
The study reveals that the rate of omicron transmission from an infected person to close contacts is high. Importantly, the study highlights that secondary transmission to household members can be avoided by immediate contact tracing, testing, and subsequent quarantine/isolation.
Moreover, the study reveals that persons with booster vaccination are much less likely to get the infection, highlighting the importance of a third vaccine dose to prevent the rapid spread of the omicron variant.