COVID-19 cases are likely to surge in winter
Respiratory viruses transmit more easily in winter just as people come together indoors
COVID-19 has affected all parts of the world, indicating that the epidemic is not limited by climate alone. Viruses, however, survive better in the environment during winter and respiratory viruses transmit more readily during a colder, drier season. While some countries experienced the peak of the pandemic around April, others are now accelerating, irrespective of weather. This points to public health measures remaining central to management of COVID-19, especially in the complex interplay with other winter pathogens.
What next
Winter in the northern hemisphere is likely to see a surge in COVID-19 cases, with current localised outbreaks expanding. Social distancing may significantly reduce the incidence of flu this year, but because symptoms of flu and COVID-19 are similar, this may nonetheless strain health systems and testing labs. The overall burden of disease and mortality may be largely driven by COVID-19, but countries past their peak may now be better placed to manage hospitalised patients, thus avoiding the devastation seen during the early pandemic.
Subsidiary Impacts
- Coming months will reveal more data on immune status of affected individuals and likelihood of reinfection.
- Initial data from advanced vaccine clinical trials will become available towards November-December.
- The political stance on public health will remain the biggest driver of reform in global management of this and future epidemics.
Analysis
In temperate regions, respiratory illnesses increase during winter months, including cold and flu viruses. This is a factor of temperature, humidity, human behaviour and changes in population immunity to specific viruses (see INTERNATIONAL: Immunity will shape pandemic's future - June 26, 2020).
While the weather has an influence on virus survival and transmission, SARS-CoV-2 infection may be influenced more by how the population behaves this winter and what non-clinical interventions are prescribed.
On average in the United Kingdom, for every 1 degree drop below 5C, there is a 10-20% increase in respiratory infections and a 1% increase in hospital admissions. Temperatures between 4-8C are the most vulnerable range.
It is estimated that the reproduction number of SARS-CoV-2 (the average number of people that each infected person transmits to) decreased from 3 to around 0.7-0.9 when lockdowns were imposed. If public health measures remain relaxed over autumn, winter may result in a surge of the epidemic.
Coronavirus patterns
Temperate regions of the world have higher temperature variations between different parts of the year. The northern hemisphere sees temperatures drop towards the end of the year, while regions in the south experience winter during June-August. Tropical regions, on the other hand, have more stable weather throughout the year.
Respiratory tract infection (RTI) prevalence data from northern and southern hemisphere countries show a 4-8% contribution of seasonal coronaviruses to winter respiratory illness, of which there are four types: HKU1, 229E, NL63 and OC43.
In the northern hemisphere, coronaviruses peak between January-March, and around August in the south (late winter in both cases).
Different coronavirus types vary in their prevalence, peaking annually or biennially. This can partly be explained by the immunity induced by a wave of infection that may prevent subsequent spread of itself or of a related virus: this is not yet known for SARS-CoV-2.
Current modelling predicts that COVID-19 outbreaks could occur at any time during the year, but the peak of severe disease may be higher in winter (for outbreaks starting in autumn or early winter).
Health and immunity in winter
Given the complexity of the immune system and other environmental/behavioural variables related to the spread of infection, it is difficult to ascertain the exact effect of climate on immunity.
RTIs can be classified into upper (URTI) or lower (LRTI), depending on where the disease manifests. SARS-CoV-2 affects both. Lower temperature, cold and dry air can lead to drying of the lining of the respiratory tract (mucosa), cell damage, inflammation and lowering of respiratory immune defences.
As a result, what would be a mild infection in summer could become relatively severe during winter. While direct systemic effects of cold weather on antibodies or T-cells is not well understood in humans, reduced sunlight and vitamin D may reduce overall immunity.
The COVID-19 death rate is currently estimated at around 1% and there is increasing evidence of lower mortality in regions such as Africa and parts of Asia, which could be due to higher pre-existing population immunity, immune status, age or genetic factors.
More conclusive data for human immunity to SARS-CoV-2 will emerge in winter from studies on people who were infected during the first wave and survived. At this point, the few confirmed cases of re-infection (from Hong Kong and the United States) have not shown disease symptoms during the second incidence, which shows that immunity is certainly protective from disease, but may still allow transmission.
Current global average exposure to COVID-19 is estimated at less than 10%, although some localised regional studies have even found around 50% seroprevalence (see INT: Local lockdowns to be key in COVID-19 management - July 16, 2020).
Virus survival and transmission
No climate type has yet inhibited transmission of SARS-CoV-2. However, the trajectory of the spread has varied between regions, but also between countries with similar climatic conditions, indicating that human behaviour and interventions are essential to the outcome.
Spending more time indoors in winter could spread COVID-19 more easily
Nonetheless, less sunlight and UV favours virus survival in the environment, while lower humidity allows respiratory droplets to remain suspended in the environment for longer.
Early modelling from China and the United States, however, has indicated only a marginal effect of the seasonal variation in temperature and humidity on transmission characteristics of the SARS-CoV-2 virus (excluding the effect of behaviour and public health measures). This means that human behaviour such as spending more time indoors, in closer contact and with less ventilation could drive the spread.
This may be mitigated by reduced mixing between households and workplaces functioning below capacity. Public health measures will therefore remain crucial to the management of COVID-19 (see INTERNATIONAL: Coming months to be pandemic's riskiest - July 30, 2020).
Interaction with other viruses
Seasonal viruses follow patterns, with early winter dominated by rhinovirus infection, followed by Respiratory Syncytial Virus (RSV) and then flu and coronaviruses towards late winter. Many are spread by children. It is still unclear whether children play a specific role in COVID-19 transmission, although they appear to become infected at a similar rate adults, albeit without progressing to disease (see INTERNATIONAL: Closing schools may be less acceptable - August 5, 2020).
Lessons from the south
While South Africa and South America saw a rapid surge in cases at the onset of winter when restrictions were eased, Australia and New Zealand have been more stable. Countries have seen surges in case numbers even after reducing them to single-digit prevalence, indicating that a small number of infected individuals can still drive outbreaks.
Increased uptake of flu vaccine and distancing measures will reduce the severity of flu this year
Meanwhile, data on influenza from Hong Kong, Australia and Japan indicated a significantly lower number of flu cases this year as compared to previous years. It is reasonable to assume that public health measures, combined with increased uptake of flu vaccine, should reduce flu.
However, due to the vast difference in hospitalisation and mortality rates, COVID-19 may still drive winter healthcare needs. In some parts of the world, this is being expected as a second wave, while for others accelerating now, winter may sustain the first peak of disease.
