Wintertime outbreaks of COVID-19 have been largely driven by whether people adhere to control measures such as mask wearing and social distancing, according toย a study published Feb. 8 in Nature Communicationsย by Princeton University researchers. Climate and population immunity are playing smaller roles during the current pandemic phase of the virus, the researchers found.
The researchers โ working in summer 2020 โ ran simulations of a wintertime coronavirus outbreak in New York City to identify key factors that would allow the virus to proliferate. They found that relaxing control measures in the summer months led to an outbreak in the winter regardless of climate factors.
โOur results implied that lax control measures โ and likely fatigue with complying with control measures โย would fuel wintertime outbreaks,โ said first authorย Rachel Baker, an associate research scholar in Princetonโsย High Meadows Environmental Instituteย (HMEI). Baker and her co-authors are all affiliated with the HMEIย Climate Change and Infectious Diseaseย initiative.
โAlthough we have witnessed a substantial number of COVID-19 cases, population-level immunity remains low in many locations,โ Baker said. โThis means that if you roll back enforcement or adherence to control measures, you can still expect a large outbreak. Climate factors including winter weather play a secondary role and certainly donโt help.โ
The researchers found that even maintaining rigid control measures through the summer can lead to a wintertime outbreak if climate factors provided enough of a boost to viral transmission. โIf summertime controls are holding the transmissibility of coronavirus at a level that only just mitigates an outbreak, then winter climate conditions can push you over the edge,โ Baker said. โNonetheless, having effective control measures in place last summer could have limited the winter outbreaks weโre now experiencing.โ
Cases have climbed in many northern hemisphere locations since November. In the United States, spikes in COVID-19 cases are thought to be tied to increased travel and gatherings for Thanksgiving and Christmas. Notably, outbreaks were recorded in temperate locations such as Los Angeles in addition to regions with much colder conditions, Baker said. At the same time, large outbreaks were observed in South Africa from November to January, which are that countryโs summer months.
โThe greater incidence of COVID-19 in various environs really speaks to the climateโs limited role at this stage,โ Baker said.
In May, the same authors publishedย a paper in the journal Scienceย suggesting that local climate variations would be unlikely to affect the coronavirus pandemic.ย The paperย suggested that hopes that the warmer conditions of summer would slow the transmission of the novel coronavirus, SARS-CoV-2, in the northern hemisphere were unrealistic.
Gabriel Vecchi, a professor ofย geosciencesย and the High Meadows Environmental Institute and co-author of both studies, said that the virus currently spreads too quickly and that people are too susceptible for climate to be a determining factor.
โThe influence of climate and weather on infection rates should become more evident โย and thus a potentially useful source of information for disease prediction โ as growing immunity moves the disease into endemic phases from the present epidemic stage,โ Vecchi said.
The most recent study provides insight on how scientists can determine the impact of various factors on the virus at various times, said co-authorย C. Jessica Metcalf, associate professor ofย ecology and evolutionary biologyย andย public affairsย ย and anย HMEI associated faculty member.
โAn important challenge that we tackle here is balancing the role of many potential factors on the trajectory of the epidemic,โ Metcalf said.ย โAs the pandemic progresses, both natural and vaccinal immunity will play an increasing role, underscoring the importance of developing a handle onย the landscape of immunity.โ
Critical factors to consider when projecting the future of COVID-19 are emerging variants of the virus, as well as how efforts to contain coronavirus have changed other diseases, said co-authorย Bryan Grenfell, the Kathryn Briger and Sarah Fenton Professor of Ecology and Evolutionary Biology andย Public Affairsย and associated faculty in HMEI.
In November, Grenfell and his co-authors in the Climate Change and Infectious Disease initiativeย published a paper in the Proceedings of the National Academy of Sciencesย that non-pharmaceutical interventions (NPIs) such as mask wearing and social distancing could result inย large, delayed outbreaksย of endemic diseases such as influenza and respiratory syncytial virus (RSV).
โThe interaction between NPIs and immunity will become even more complex as a variety of vaccines are deployed and new viral variants arise,โ Grenfell said. โUnderstandingย the impact of these variables underlines the importance of immune surveillance and greatly expanded viral sequencing.โ
Additional authors of the current paper includeย Wenchang Yang, an associate research scholar in geosciences at Princeton.
The paper, โAssessing the influence of climate on wintertime SARS-CoV-2 outbreaksโ was published online Feb. 8 by Nature Communications. This work was supported by theย Cooperative Institute for Modelling the Earth Systemย (CIMES), the High Meadows Environmental Institute (HMEI) and theย Princeton Institute for International and Regional Studiesย (PIIRS).
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