SARS-CoV-2 pandemic: Why do most viral epidemics spread cyclically in autumn and winter in the globe’s temperate regions?

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Why do most viral epidemics spread cyclically in autumn and winter in the globe’s temperate regions?

According to an interdisciplinary team of researchers of the Italian National Institute for Astrophysics, the University of Milan, the Lombardy regional agency for the environment and the Don Gnocchi Foundation, the answer is intimately related to the sun.

Their theoretical model shows that both the prevalence and evolution of epidemics are strongly correlated with the amount of daily solar irradiation that hits a given location on the Earth at a given time of the year.

The work of the Italian team was recently published in the iScience journal.

“Our model offers a simple answer to an important, yet still unsolved, scientific question,” says Fabrizio Nicastro, INAF researcher and PI of the work.

“Why do many viral respiratory epidemics, such as influenza, develop cyclically during autumn and winter only in the temperate regions of the globe’s northern and southern hemispheres, while they seem to be present at all times – albeit with lower prevalence compared to the seasonal cycles in the temperate regions—in the equatorial belt? And what triggers and determines such seasonality?

In our work, we propose that what causes the seasonality of airborne-transmitted epidemics is exactly the same mechanism that causes seasons on our Planet: the amount of daily solar irradiation on the Earth.”

It is well known that ultraviolet (UV) light is able to deactivate viruses and bacteria of many different kinds. The solar UV light that reaches the Earth must therefore have some disinfecting power on the exposed parts of the Planet.

The efficiency of the UV deactivation of a particular virus or bacterium depends on the virus or bacterium itself, but, for a given location on Earth, it is undoubtedly greater when the solar irradiation is stronger (summer) and lower when the solar irradiation is weaker (winter).

Such cyclicality of the solar disinfecting action, with annual frequency, is able to constructively resonate with another frequency typical of epidemics: the loss of immunity of the virus’s host due to its antigenic shift/drift.

The combination of these two mechanisms triggers the seasonality of epidemics, on timescales that range from a few years to tens of years, depending on the antigenic frequency.

The role of the Sun in the spread of viral respiratory diseases
Evolution of Influenza-like epidemic. Credit: Paolo Bonfini, University of Crete

The model proposed by the Italian researchers reproduces the seasonality observed in different locations of the Earth accurately for epidemics with an intrinsic reproductive number (R0) lower than about 2 – an influenza typically has R0~1 – and is also able to model epidemics with a much larger intrinsic reproductive number, such as the current SARS-CoV-2 pandemic with R0≈3-4.

These models predict high-intensity intermittent initial cycles, which eventually stabilize (on timescales that depend on the antigenic-shift frequency) onto seasonally-synchronized, moderate-intensity annual cycles.

“From an epidemiologic point of view, these models clarify an important and long-standing mystery: why do influenza epidemics disappear every year when the number of susceptible individuals is still very far from that needed to trigger the herd immunity mechanism?” adds Mario Clerici, Immunologist at the University of Milan and the Don Gnocchi Foundation.

“The Italian data of the SARS-CoV-2 pandemics can also be described accurately by our model—concludes Nicastro—but the predictive power of the model depends critically (other than on the implementation of new restriction measures) on the exact UV-B/A lethal doses for the COVID-19 virus, which our collaboration is about to measure.”


Even in nonpandemic years, influenza and other etiologies of pneumonia represent the eighth leading cause of death in the United States, and respiratory viruses are the most commonly identified pathogens among hospitalized patients with community-acquired pneumonia (1). Epidemics of seasonal influenza occur on an annual basis.

In the United States, the 2019–2020 seasonal influenza epidemic resulted in tens of millions of cases—the majority of which occurred before the coronavirus disease 2019 (COVID-19) pandemic surged. Now, COVID-19, caused by the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is an ongoing pandemic that has strained and, in some locales, overwhelmed health care systems.

What can we expect as the COVID-19 pandemic evolves and seasonal influenza comes again? How can the epidemiology and biology of these infections inform our preparation strategies?

The last influenza pandemic, caused by the then-novel H1N1pdm09 virus, began in the spring of 2009 and caused an estimated 61 million cases, 274,000 hospitalizations, and 12,500 deaths in the United States over the following year (2).

Despite its inclusion in the influenza vaccine since 2010, H1N1pdm09 continues to circulate annually in the community and was the predominant influenza A virus strain during the 2019–2020 influenza epidemic. In contrast, the last human coronavirus epidemic, SARS (caused by SARS-CoV), abated because of aggressive containment procedures before a vaccine could be deployed; community transmission of SARS-CoV has not occurred since 2004.

Based on the course of the COVID-19 pandemic to date and anticipated vaccine development timelines, it is clear that SARS-CoV-2 will not follow the abruptly terminating trajectory of SARS-CoV. Rather, it is likely that community transmission of SARS-CoV-2 will continue as we enter the next influenza epidemic. Several factors, at least in part, will determine the overall severity of the upcoming respiratory virus season and can inform how we prepare:

1) Transmission. Influenza viruses and SARS-CoV-2 predominantly spread via respiratory droplets that are transmitted during close community contact. Consequently, social distancing policies designed to limit COVID-19 transmission are also effective against influenza (3). The corollary is that if COVID-19 cases begin to spike in the fall of 2020, retightening of social distancing measures could allay early spread of influenza to flatten the case-rate curves for both diseases.

2) Vaccination. How closely the influenza antigens included in the annual vaccine match the viruses that circulate in the coming season determines the vaccine’s efficacy. For 2019–2020, influenza vaccine effectiveness was 45% in the United States—a level comparable to years in which close vaccine antigen-circulating strain matching occurred (4). Influenza vaccine coverage for the previous season was only 45% among adults in the United States, despite the recommendation for universal vaccination (5). Thus, as we await SARS-CoV-2 vaccine trials, plans to mitigate the overall burden of respiratory disease should include efforts to increase rates of vaccination against influenza, particularly among older adults who have increased susceptibility to both influenza and COVID-19.

3) Co-infection. Co-infection with another respiratory pathogen, including influenza, occurred in more than 20% of SARS-CoV-2–positive patients who presented with a respiratory viral syndrome early in the COVID-19 pandemic (6). The potential for co-infection carries diagnostic implications, as detection of an alternative etiology for a respiratory syndrome cannot be used to exclude COVID-19 in areas where SARS-CoV-2 testing remains limited. Clinicians may also need to modify therapeutic regimens depending on the specific copathogen (e.g., oseltamivir for influenza). These findings underscore the need for widespread availability of rapid diagnostics for SARS-CoV-2 and other respiratory pathogens.

4) Disparities. African Americans, Latinx, and Native Americans are overrepresented among COVID-19 cases and deaths (79). Also, disparities in influenza vaccination rates have historically existed among minority populations (10). The etiologies for health care disparities are complex and longstanding. The COVID-19 pandemic has highlighted these unconscionable disparities and must galvanize public health efforts aimed to limit viral transmission, increase vaccination rates, deploy rapid diagnostics, and expand other health care services for vulnerable populations, including communities of color, the poor, and older adults. Beyond these public health initiatives, overcoming disparities related to influenza and COVID-19—and disparities in health outcomes more generally—will require major societal change to address the longstanding racial and economic inequities that affect comorbidities, infection risk, and access to timely, high-quality health care.

Humans have suffered from influenza for millennia, and we can expect that the new reality of COVID-19 will only complicate the next influenza season.

Measures to reduce the overall burden of respiratory viral infection – including social distancing, increased vaccination rates, availability of diagnostics, and addressing health care disparities – are paramount in planning for the months ahead. Moreover, careful evaluation and modification of these factors will enhance preparedness ahead of viral pandemics yet to come.

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More information: Fabrizio Nicastro et al, Forcing Seasonality of Influenza-like Epidemics with Daily Solar Resonance, iScience (2020). DOI: 10.1016/j.isci.2020.101605

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