New Year's Eve celebrations linked to spike in COVID-19 cases: Small gatherings, not large events, the main culprit

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A recent Scientific Reports study explores whether New Year’s Eve celebrations impacted transmission of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causal agent of the coronavirus disease 2019 (COVID-19), in Belgium.

Study: Unravelling the effect of New Year’s Eve celebrations on SARS-CoV-2 transmission. Image Credit: fornStudio / Shutterstock.com Study: Unravelling the effect of New Year’s Eve celebrations on SARS-CoV-2 transmission. Image Credit: fornStudio / Shutterstock.com

Background

Several studies have reported an increase in COVID-19 cases following public holidays and celebrations; however, to date, no study has identified a single causal evidence.

Existing data has shown that superspreading events (SSEs) are one of the significant contributors to the transmission of SARS-CoV-2 and other pathogens. Thus, it is important to understand the exact role of SSEs in increasing the spread of SARS-CoV-2 during holidays.

About the study

The current study investigated whether New Year’s Eve celebrations in Belgium led to a higher incidence of SARS-CoV-2 infections, particularly in the local community of higher education students.

The Belgian government implemented stringent restrictions on social interactions between November and December 2021. This policy was implemented based on their anticipation of an overwhelming surge in SARS-CoV-2 Omicron cases.

Several other restrictions were implemented by the end of December, including wearing face masks in public indoor areas, prohibiting indoor mass gatherings, school closures, and mandatory remote work in several professional sectors.

Christmas and New Year celebrations were restricted to private gatherings in residential locations during this time. However, no limit was imposed on the number of individuals allowed to be present at these gatherings.

Even after the implementation of these restrictions, an increase in COVID-19 cases was documented after New Year’s Eve, 2022. This finding was based on the trace program for higher education students in the city of Leuven.

Based on contact tracing, one previous study estimated the mean incubation period of the Omicron BA.1 lineage to be 3.03 days. Using phylogenetic and contact tracing data, the current study estimated how NYE celebrations influenced Omicron case numbers in higher education students in Leuven, Belgium.

About 50,000 higher education students at KU Leuven Association in Belgium were considered for the analysis. The epidemic curve was plotted based on self-reported symptoms.

Study findings

During the study period, Omicron BA.1 was the dominant circulating strain of SARS-CoV-2. Importantly, over 90% of students were vaccinated with a mean of three months from the last vaccination dose.

Based on test and trace program results, a short-lived surge in COVID-19 cases was observed after the holiday celebrations at the start of 2022. Out of 50,000 higher education students considered in the study, 2,505 underwent a polymerase chain reaction (PCR) test within the first two weeks of the new year.

Among tested students, 16.9% were positive for COVID-19. Excluding cases with weak signals or spike (S)-gene target failure, the remaining 286 individuals were considered for further experiments and were interviewed by the contact tracing team. Subsequently, 54 individuals were grouped under possibly infected on NYE, 36 cases under possibly infected on NYE, and 196 cases were categorized under unknown sources. 

A strong association was observed between social gatherings on NYE and increased COVID-19 cases among higher education students. More specifically, small-scale social gatherings on NYE were associated with a rise in COVID-19 cases.

Although there was a high secondary attack rate, SSEs were responsible for 16-34% of infections on NYE. Thus, small-scale events, rather than SSEs, were the main contributors to a surge in new cases of COVID-19. 

First-generation infections primarily originated from small-scale social gatherings. This observation could be true for both national and international scenarios regarding the transmission of the Omicron BA.1 strain.

The winter holidays include Christmas and New Year celebrations, both of which are linked with two occasions of large-scale inter-generational and inter-regional transmission. Family members and friends often celebrate in enclosed spaces with physical interaction and strong vocalization during these gatherings.

The resulting infections from these events have little opportunity to spread further until schools and workplaces reopen. Consistent with previous studies, the current study estimated the incubation period of the Omicron strain to be about 2.7 days.

Conclusions

The current study is the first to indicate a robust link between public holiday celebrations and an increase in SARS-CoV-2 cases. Rather than mass gatherings, simultaneous celebrations in small groups during the winter holidays were found to be the main contributor to increased SARS-CoV-2 infections.

Journal reference:
  • Geenen, C., Thibaut, J., Laenen, L., et al. (2023) Unravelling the effect of New Year’s Eve celebrations on SARS-CoV-2 transmission. Scientific Reports 13(1);1-9. doi:10.1038/s41598-023-49678-x
Dr. Priyom Bose

Written by

Dr. Priyom Bose

Priyom holds a Ph.D. in Plant Biology and Biotechnology from the University of Madras, India. She is an active researcher and an experienced science writer. Priyom has also co-authored several original research articles that have been published in reputed peer-reviewed journals. She is also an avid reader and an amateur photographer.

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