Multistate Outbreak of SARS-CoV-2 B.1.1.529 (Omicron) Variant Infections Among Persons in a Social Network Attending a Convention — New York City, November 18–December 20, 2021

Investigation and Findings

Through a case investigation interview with patient A on November 30, contact tracing,^§ and collaboration with various state and local health departments, the 29 attendees who had close contact with patient A during the event were identified. A questionnaire was developed to collect demographic, epidemiologic, and clinical information for the index patient and the 29 close contact attendees. In addition, a second questionnaire was developed to collect similar information from household contacts of any of these attendees (including patient A) with confirmed SARS-CoV-2 infection. During November 30–December 20, 2021, investigators conducted interviews with patient A, close contact attendees, and household contacts who did not attend the convention. This activity was reviewed by CDC and was conducted consistent with applicable federal law and CDC policy.^¶

Among the 30 attendees (patient A and their 29 close contacts), 23 (77%) residing in 13 states** were interviewed; 12 (52%) were men, and the median age was 24 years. All 23 attendees had received a primary COVID-19 vaccination series (135–281 days before convention),^†† with 11 (48%) having received a booster dose (six and five received the booster ≥14 days and <14 days, respectively, before the convention); one person reported a history of COVID-19 (Table 1).

Upon notification, all 23 interviewed attendees sought testing (15 within 2 days among 19 with known first test date), and 18 (78%) used one or more at-home antigen tests. Positive SARS-CoV-2 test results were received by 16 persons in 10 states^§§ (attack rate = 70%), including two of three who used at-home antigen tests only, three of five who used laboratory-based tests only, and 11 of 15 who used both at-home antigen and laboratory-based tests. Five attendees’ specimens (from five states) were sequenced and were identified as Omicron variant sublineage BA.1 with no discernable difference. In addition, three household members of a single attendee had this same Omicron sequence confirmed, but the convention attendee in this household used an at-home antigen test only, and no sequence confirmation was available. Among 11 (48%) of the 23 interviewed attendees who had received a COVID-19 booster dose before the convention, six received a positive test result. Among 12 (52%) attendees who had not received a booster, 10 received a positive test result. All infected attendees reported at least one symptom (median = five) with symptom onset during November 21–26; seven reported symptom onset within 2 days of the final day of the convention. The median duration of symptoms was 11 days; the most commonly reported symptoms included nasal congestion, fatigue, cough, and sore throat. The median incubation period was 2–5 days (based on earliest and latest exposure). No hospitalizations or deaths were reported.

Overall, 21 (91%) of 23 interviewed attendees participated in the convention all 3 days. Twenty (87%) attendees reported mask-use “always” and three (13%) reported mask-use “sometimes” while at the convention (mostly removing mask to eat and take pictures). Attendees engaged in activities outside the convention, including karaoke, eating indoors, and visiting bars/clubs (Table 1). Among 14 interviewed attendees who stayed at accommodation A, nine received a positive test result, as did seven of nine who stayed in other accommodations (Figure). No attendee reported international travel before the convention.

Upon return home, the 16 attendees with positive test results exposed 20 household contacts (median age = 55 years) during their infectious period. Among household contacts, 11 (55%) were male, and none reported a previous history of COVID-19 (Table 2). Nineteen (95%) household contacts were fully vaccinated, and 10 (50%) had received a booster dose 2–94 days before exposure to the attendee (median = 27 days). Among 18 household contacts tested following exposure to an attendee with a positive SARS-CoV-2 test result, six (33%) received a positive result, including four who had received a booster dose >14 days before exposure. Symptom onsets ranged from November 26 to December 7 (median symptom duration = 13 days). Four patients reported fewer than five symptoms; the most commonly reported symptoms were nasal congestion, fatigue, cough, runny nose, and change in taste. The median incubation period was 1–5 days (based on earliest and latest exposure). No hospitalizations or deaths were reported.

Public Health Response

On December 2, 2021, MDH notified respective state and local health departments of four convention attendees with close contact to the index patient regarding potential exposure to the SARS-CoV-2 Omicron variant. That same day, CDC issued an Epidemic Information Exchange (Epi-X) to U.S. health departments to identify COVID-19 cases among event attendees. On December 6, CDC requested permission to conduct interviews with the index patient, close contact attendees, and their households; additional state and local health departments were notified as other close contacts were identified.

Discussion

In this group of convention attendees with several days of close contact, SARS-CoV-2 Omicron variant attack rates were high, both among the attendees (70%) and among household contacts of infected attendees (33%). Among 16 cases that occurred within this attendee cluster, all persons had been vaccinated, as were the six household contacts who became infected. The high attack rates among fully vaccinated persons illustrate Omicron variant’s partial escape from vaccine-induced immunity (2); however, illness was relatively mild among this cohort, consistent with evidence that vaccinated persons with infections are less likely to experience serious illness. Potential contributing factors to the high attack rates include unmasked and prolonged contact in social settings and residential accommodations. This finding among this group contrasts with the observed overall transmission at the convention, which was relatively low; all Omicron cases identified from the larger investigation were associated with this group of close contact attendees. The low overall transmission at the convention is likely because of short interactions among participants in less confined space combined with high vaccination coverage, high prevalence of mask use, and high-efficiency particulate air filtration (1).

Among this attendee cluster with close unmasked contact and known booster status, a lower proportion of those who had received a COVID-19 booster dose received a positive test result (six of 11) compared with those who had not received a booster dose (10 of 12). Although this attendee cluster was small, the finding that prevalence was lower among those who had received a booster is consistent with other study findings (2–4). Vaccination, including booster doses,^¶¶ reduces risk for infection; persons who have received a booster dose can still become infected, but they are less likely to experience severe illness, hospitalization, or death.

The findings in this report are subject to at least four limitations. First, testing for statistical significance was not performed because of the small sample size. Second, high use of at-home testing and time between positive test results and this investigation limited the availability of residual samples for sequencing; infections without sequencing data might have been caused by other variants. Third, seven close contact attendees never responded to requests for interview, which could have biased results. Finally, because at-home tests might have lower sensitivity than RT-PCR tests,*** some infections might have been undetected.

Early notification and testing with at-home antigen tests resulted in immediate quarantine or isolation and early diagnoses for the persons in this cluster, which might have reduced secondary attack rates in these households (5). Patient A notified the group after learning that another attendee was infected, 1–4 days after possible exposure, prompting contacts to seek testing, monitor for symptoms, and isolate or quarantine. The high use of at-home antigen tests suggests that persons with access to these tests will use them and share results. Although at-home antigen tests might have lower sensitivity than RT-PCR tests, broad access might result in earlier detection and notification, thus assisting in interrupting transmission. This investigation reinforces the importance of COVID-19 booster doses and early notification in combination with other multicomponent prevention measures to limit transmission and prevent severe illness from Omicron and other SARS-CoV-2 variants.