Understanding the role of seasonality in England’s lockdown exit strategy

Even though the impact of vaccinations on transmission, serious illness and death has rightfully grabbed the headlines, there is another element central to England’s lockdown exit strategy. You often hear people talk about it and it does have significance for resource allocation and setting for public health policy, but it remains a poorly understood concept. It is, of course, seasonality.


While researching the topic, I came across a journal article by David Fisman (2012), a Canadian epidemiologist at the University of Toronto’s Dalla Lana School of Public Health, which included this succinct definition of seasonality:

“In broad terms, seasonality represents oscillation in pathogens’ effective reproductive number, which, in turn, must reflect oscillatory changes in infectiousness, contact patterns, pathogen survival, or host susceptibility.

Whereas a dictionary definition might simply suggest that a disease is seasonal if peak incidence is correlated with a particular period of the calendar year, an alternative definition is ‘the state of recurring at regular intervals’, and, indeed, the seasonality of infectious diseases usually represents a periodic process: one that has sinusoidal peaks and troughs at regular intervals”.

David Fisman

In laymen terms, seasonality thus refers to a virus’s varying ability to infect people based on seasonal factors, such as infectiousness, contact patterns, pathogen survival, or host susceptibility. This typically leads to an up-and-down oscillation with peaks and troughs happening at regular intervals. The most common example used is a winter peak and a summer trough. We see this happen most years with influenza, for example.

Central to the concept of seasonality, therefore, is a disease’s ‘reproductive number’: the average number of secondary cases produced by a primary infective case. For example, a reproductive number of 3 means that 1 person has gone on to infect 3 people. However, there are two types of reproductive number, both of which are important to understanding seasonality. They are the ‘basic’ and ‘effective’ reproductive number.  

While the ‘basic’ reproductive number refers to the reproductive number when everyone in the population is susceptible to infection (i.e. no one has prior immunity), the ‘effective’ reproductive number is the ‘basic’ reproductive number multiplied by the fraction of the population susceptible to infection. This means that as the number of people catching a disease increases, the fraction of the population susceptible to infection decreases, pulling the effective reproductive number down. We often see talk about the effective reproductive number in debates about the level of immunity acquired in the population.

Here is a diagram of the equation for the ‘effective’ reproductive number:

Fisman goes on to argue that “seasonality depends on either periodic fluctuation in ‘basic’ R, or periodic fluctuation in population-level susceptibility”, making both components of the equation important. He states that the factors that affect ‘basic R’ are (1) contact rates in the population; (2) the transmissibility of pathogens per infectious contact; and (3) duration of infectivity or environmental survival of pathogens. Alternatively, the factors affecting population-level susceptibility are (1) host effects; and (2) drift in viral antigens.

This shows that when talking about seasonality, there are many factors to consider. Fisman summarises these as “drivers” in a useful table, each with an example:     

All of these appear relevant to COVID-19 except maybe the second one (“seasonal increase in insect vector density and/or biting rate; increased density of reservoir animals”) as this is not the principal way people are infected with the disease.


Given COVID-19 has only been around for over a year, understanding the role of seasonality on its virulence, transmissibility, and genetic adaption is difficult because it is still a relatively new virus. Thus, scientific literature so far is based on a combination of two things: (1) the seasonal effects of similar viruses; and (2) analysis of scant data. As time progresses, these analyses will improve, along with our understanding of the impact of seasonality on COVID-19, but until then it is important to remember that there is still a lot we do not know about the virus and that will continue to be the case even after we have vaccinated our whole population.

Initial research projected that, without intervention, COVID-19 would decrease temporarily during last summer, rebound by autumn, and peak in the winter. In October, NERVTAG (The New and Emerging Respiratory Virus Threats Advisory Group) – the UK government’s advisory body that “provides scientific risk assessment and mitigation advice on the threat posed by new and emerging respiratory viruses and on options for their management” – released a document summarising its findings on why this would be the case based on an evaluation of the available literature. It argued that four factors were likely to combine to exacerbate the COVID-19 epidemic during the winter months:

  • The continued susceptibility of the population
  • The direct effect of environmental variables (such as temperature and UV light)
  • The indirect effect of poor weather leading to people spending more time indoors
  • Other seasonal changes in contact rates, such as school opening and seasonal festivals

Each finding included a level of confidence (low, medium, or high) which correlated to the strength of the evidence backing up each finding. Below is a table collating these findings:

What it shows is that NERVTAG was most confident on susceptibility having a much bigger impact on transmission than environmental factors, though they did believe indirect effects (decrease in ventilation rate during the winter and increased time spent indoors) were likely to play a more significant role in facilitating transmission during winter than the direct effect on seasonal changes in temperature and UV light. This meant the important factor to control related to contact rates in the population (how often people came into contact with others), which is why they supported a national lockdown as it was effective at decreasing contact rates.  


Well, central to NERVTAG’s argument was the role of outdoor vs. indoor on transmission. They conceded that “very little transmission occurs outdoors”, whereas “the indoor environment [is] where the vast majority of transmission occurs”. In winter, this posed a significant problem as individuals would spend longer indoors due to poor weather. Yet the reverse is true for summer; better weather encourages people to spend more time outdoors, which is a safer environment because transmission is low. This is relevant to our exit strategy from this third lockdown.

While waiting for immunity to be built up in the population through vaccinations, prioritising outdoor social interaction makes sense because it poses a minimal risk for transmission. Yet, currently, you can only meet one other person outside for exercise. Early reports into possible relaxations suggest that, in the first easing of restrictions (8th March), the government is only planning to expand the purpose for meeting one friend from solely exercise to exercise and/or socialisation. If accurate, this seems to be a missed opportunity to take advantage of the benefits of outdoor interaction.

The outdoors provides many aspects, from socialisation and single-person exercise, to group sports and outdoor play. Allowing people to utilise these various aspects sooner rather than later, in both March and April for example, would increase enjoyment and compliance as people’s quality of life will improve. It would also provide an opportunity to stress the importance of outdoors activity, laying the groundwork for a prioritisation of outdoor hospitality and larger outdoor events as summer draws in.

People often use the counter that the public will exploit these rules. However, a transparent conversation about the benefits of outdoors activity is possible and worthwhile. If alcohol and reduced inhibitions is really that much of a concern, banning alcohol out in public seems like a fair response that would still allow the majority to enjoy the benefits of outdoors in a safe manner. The distinction between outdoors and indoors is what is important moving forward, so showing the public that outdoor activity is safe while indoor activity is not is the best way to maintain compliance as we move into the next phase of our recovery from coronavirus.


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