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  • Many viruses are more active during winter.
  • COVID-19 rates have spiked in the winter months in the Northern Hemisphere and in summer months in the Southern Hemisphere.
  • A new study suggests that the current winter peaks result from relaxing control measures — not changing climatic conditions.

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Scientists suggest that the rate of infections with SARS-CoV-2, the virus responsible for COVID-19, during the winter is more likely to be influenced by relaxed control measures than any climatic conditions, such as cold weather.

The research, published in the journal Nature Communications, may be valuable in understanding how best to protect against COVID-19 as the disease potentially shifts from pandemic to endemic status.

Study co-author Gabriel Vecchi, a professor of geosciences at the High Meadows Environmental Institute, at Princeton University, in New Jersey, says:

“The influence of climate and weather on infection rates should become more evident — and thus a potentially useful source of information for disease prediction — as growing immunity moves the disease into endemic phases from the present epidemic stage.”

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Many viruses are more active in winter than summer, particularly in temperate climates.

Scientists are beginning to understand why this might be, and the reasons are different for each virus.

In temperate climates, these reasons concern how well the virus survives at a particular temperature and humidity level, the capacity of the human viral defense system, which includes the nose and immune system, and how humans behave in cold temperatures.

The incidence of COVID-19 greatly increased during the winter months of 2020 to 2021 in the Northern Hemisphere. But it has been unclear whether this was due to the changing weather — as with seasonal viral illnesses such as influenza — or some other factor.

In the summer of 2020, the authors of the present study, a team from Princeton, simulated an outbreak of a coronavirus in New York during winter.

With this simulation, they were able to predict the effects of climate on SARS-CoV-2, particularly in the context of changing control measures such as mask-wearing and social distancing.

The researchers used the endemic betacoronavirus HKU1 as a stand-in for SARS-CoV-2. Their previous work had found that this betacoronavirus is more sensitive to climate than SARS-CoV-2.

Therefore, the effects of climate on the spread of COVID-19, as predicted by this model, are likely to be the upper limit of what may actually occur at this point in the pandemic.

The researchers found that although the climate is likely to have some effect on the rate of SARS-CoV-2 transmission — it is far less significant a factor than control measures in the months leading up to winter, given the number of people who are still susceptible to the virus and how well the measures are implemented.

The researchers used New York as the basis for their modeling due to good access to past and current climate and COVID-19 data.

They found that the winter peak was most likely influenced by five factors. The first concerned the effectiveness and implementation of nonpharmaceutical interventions (NPIs) such as mask-wearing and social distancing.

The researchers estimated an effective NPI implementation of 35% but found that if this increased to 55%, these measures were better able to override climatic conditions.

The second and third factors concerned the duration of immunity and the accuracy of reporting. In the United States, only 10% of infections are currently being identified.

The final two factors concern how much the weather varies each year and how sensitive the virus is to climatic conditions such as humidity and heat. At present, researchers know that SARS-CoV-2 is more stable in cold, dry conditions.

According to the study’s lead author, Dr. Rachel Baker, a research associate at the institute, “Our results implied that lax control measures — and likely fatigue with complying with control measures — would fuel wintertime outbreaks.”

“Although we have witnessed a substantial number of COVID-19 cases, population-level immunity remains low in many locations. This 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 don’t help,” she adds.

The researchers note that climate may have more of a significant effect in situations where the virus is only just being kept under control.

Dr. Baker explains, “If summertime controls are holding the transmissibility of [the novel] coronavirus at a level that only just mitigates an outbreak, then winter climate conditions can push you over the edge.”

“Nonetheless, having effective control measures in place last summer could have limited the winter outbreaks we’re now experiencing.”

The lead author also notes that the limited effect of climate is reflected in the high rates of infection during winter throughout the U.S. — despite the significantly different climatic conditions across the country.

“The greater incidence of COVID-19 in various environs really speaks to the climate’s limited role at this stage,” she observes.

The researchers conclude that the virus is currently able to spread too quickly for the climate to play a significant role in reducing infections.

They write:

“In all cases, if susceptibility is high and NPI measures are reduced, large outbreaks will occur — no matter the climate conditions.”

But in the future, as the susceptibility of SARS-CoV-2 alters due to vaccination and natural immunity from infection, the role of control measures and climate is likely to become more finely balanced, they note.

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