Right now, India is completing more than a month of national lockdown and moving toward some loosening of restrictions in less-infected areas. The implications of stopping the movement and activity of 1.3 billion people would be hard to overstate. Ironically, one of the many side effects of the policy has been the migration of millions of poor people who lacked the resources to shelter in place for the duration of the lockdown. Indian Prime Minister Narendra Modi faced a terrible choice: lock down the country with massive economic and social repercussions, or leave the country open and allow an outbreak of perhaps unparalleled scale in the history of the world.

How will the government continue to adjust its strategy in the coming months? In some ways, the harmful consequences of lockdown are more obvious than the successes, as we will never know how the virus could have spread in the absence of the lockdown. Given that COVID-19 was not eradicated in India during the lockdown, an easing up in May, June, and July without robust surveillance risks igniting the pandemic. This begs the question of whether India can remain in a partial lockdown. If so, for how long without catastrophic economic and social damage? How does a country like India weigh the implications and timing of lockdowns to achieve the maximum containment with the fewest side effects?

How does a country like India weigh the implications and timing of lockdowns to achieve the maximum containment with the fewest side effects?

Of course, India is only a single example of a country in lockdown, but the dilemma is the same elsewhere—particularly for lower-income countries where the poor lack reserves to shelter in place and the economic privileges to practice social distancing when 190 people can share a single toilet. Countries and political leaders are between a rock and a hard place. The number of official COVID-19 cases in India is small as a percent of the population: fewer than forty thousand as of the beginning of May. In African countries contemplating lifting lockdowns, the number of cases could be less than one hundred. Are these countries “ticking time bombs” or will they have different epidemiological trajectories than colder countries in North America? This situation poses an unanswered trillion-dollar policy question: how significant is climate for COVID-19 transmission, and how will climate affect transmission across different geographic regions?

Could COVID-19 Be Seasonal?

Many infectious diseases show seasonal outbreak patterns. The well-known flu season in the Northern Hemisphere gets its name from the episodic and seasonal pattern of influenza outbreaks that occur during winters.

For human influenza viruses, studies suggest absolute humidity affects both transmission as well as virus survival

Although not all infectious diseases peak in the winter, a study in the United Kingdom examining four human coronaviruses did indeed show seasonality consistent with influenza. This is not to suggest COVID-19 will follow this pattern, as novel viruses often display different characteristics than endemic ones. The reasons for seasonality are complex. Seasonal impacts vary across diseases but can range from pathogen stability between hosts to changes in immune function based on environmental characteristics. For human influenza viruses, studies suggest absolute humidity affects both transmission as well as virus survival. Complicating the picture, the flu exhibits different transmission patterns in temperate and tropical climates. An analysis of seventy-eight individual studies concluded that while transmission occurred in temperate regions in cold-dry conditions, transmission occurred in tropical regions in humid-rainy conditions.

Does the Case of Singapore Refute COVID-19’s Seasonality?

The World Health Organization (WHO) clearly states in its advice for the public that “from the evidence so far, the COVID-19 virus can be transmitted in ALL AREAS, including areas with hot and humid weather. Regardless of climate, adopt protective measures if you live in, or travel to an area reporting COVID-19.”

While this does not explicitly say seasonality is irrelevant in COVID-19 transmission, it does imply that public policy or social practice should not be different in different climates. With more than seventeen thousand confirmed cases and a temperature ranging from the high eighties to low nineties Fahrenheit in January and February 2020, the case of Singapore appears to show transmission could be unaffected by heat and humidity. However, Singapore’s late Prime Minister Lee Kuan Yew once described air conditioning as “the greatest invention of the twentieth century,” and the heat and humidity of Singapore do not represent the conditions in which most Singaporeans live. Thus, the experience of Singapore may not be the same as lower-income countries with similar climates but less time spent indoors in climate-controlled environments. The low number of cases in other major tropical urban areas is also puzzling if the transmission is not affected by heat and humidity.

Other countries provide similarly interesting case studies. As of May 2, 2020, Vietnam reported only 270 cases out of a population close to 100 million. This remarkably low number of cases is despite its geographic proximity and economic ties to China. Even if there are a significant number of unreported cases, the clinical presentation of epidemics in places such as New York are so obvious as to not require complete testing to understand the magnitude of the problem. As Vietnam begins to open up, this again raises the question of whether places such as Vietnam could still be “ticking time bombs,” with hidden epidemics and people dying at home. Or will they not follow the trajectory of many regions with cooler and drier climates because of the potential climate effect on transmission? Will the outbreaks in such countries follow the patterns typical of the flu in tropical regions?

Cases such as Singapore and Vietnam are illustrative of the broader problem of finding the signal amidst the noise. The noise in the data that makes it hard to determine the impact of seasonality and the impact of climate on coronavirus transmission comes from a number of variables.

What to Look for in the Data

As the Northern Hemisphere transitioned from winter to spring, scientific commentary became increasingly geared toward expecting a reduction in the transmission rate as a function of warmer temperatures. Although this assumption could have a valid basis, one should be careful constructing causal relationships between environmental variables and the appearance or disappearance of the virus given the many confounding factors such as those listed above.

Human coronaviruses exhibit a seasonal pattern, as outbreaks tend to be associated with cooler and drier conditions

In general, human coronaviruses exhibit a seasonal pattern, as outbreaks tend to be associated with cooler and drier conditions. If COVID-19 follows patterns similar to the flu, the effect of climate could be different in tropical regions than in temperate ones. Virologists and epidemiologists are still debating the upper and lower thresholds regarding the behavior and survivability of the virus. A scan of current literature suggests an optimal habitability zone for the SARS-CoV-2 virus (which causes COVID-19) to fall between 5 and 11°C (41 and 51.8°F), with low relative and specific humidity. When the outbreak first started being widely reported in China, the hospitalization rates in northern cities were much higher than those reported from cities at lower latitudes. However, additional studies point to habitability ranges with higher upper temperature thresholds.

Based on these parameters, many epidemiologists expect that as temperatures and atmospheric humidity begin to rise in sync with typical seasonal patterns, the rate of infections should begin to decrease in the Northern Hemisphere. As the world is dealing with this global crisis in real time, unlike other forms of transmissible disease, there simply is not sufficient observational data to construct more precise mathematical models, so expected ranges associated with seasonality could be the best available guide.

Empirical data is being collected and tested each day, and the accuracy of the representative nature of this data is still questionable. However, disease modelers still do not have access to a long enough history of data to construct a proper out-of-sample verification process. Therefore, while models will be useful, their output should be used with caution.

Why Seasonality Matters If True

If COVID-19 does show strong seasonal patterns in which heat and humidity act to impede transmission, these effects should be recognized. Not accounting for seasonal characteristics when they exist could lead to situations such as

• country complacency and relaxing of social-distancing norms resulting from mistakenly attributing the decline in new cases to effective policy action versus normal fluctuations with seasons;
• shutting down schools either before or after the optimal time, recognizing that closing schools for the duration of the epidemic will result in other harm;
• mistiming national lockdowns (or regionally differentiated lockdowns within countries) that limit viral transmission but also cause economic harm and hence cannot be implemented for the duration of the epidemic;
• inadequately distributing essential resources such as ventilators and personal protective equipment based on incorrect models; and
• implementing uniform social-distancing practices across indoor and outdoor spaces when transmission could be less likely to occur in places such as parks than in shopping malls.

More generally, knowing the natural fluctuations of disease transmission will help in the inevitable policy interplay occurring between periods of broad clamping down, which are effective and costly, and more open periods consisting of surveillance and tracing.

The Burden of Proof for COVID-19

In some legal systems there is a distinction between the level of evidence required for criminal and civil judgements. In U.S. criminal cases the prosecution must show that the defendant is guilty “beyond a shadow of doubt.” In civil cases, meanwhile, the bar is set lower—to a preponderance of the evidence. By way of analogy, in the case of COVID-19, the scientific community is looking for evidence of seasonality that is “beyond a shadow of doubt,” whereas policymakers are making decisions based on a preponderance of evidence. Importantly, though, most of those policymakers are assuming that there is no effect of climate on transmission. Although there is not strong evidence to suggest a climate effect, neither has a climate effect been shown to be false. Thus, assuming no climate effect could be the correct presumption, but it could also be proven wrong.

If evidence arises that climate is important, it needs to be rapidly incorporated into models in order to feed into policy decisions

In the 2003 Severe Acute Respiratory Syndrome (SARS) outbreak, the vast majority of scientific publications occurred after the epidemic was over. This and other forces led to discussion of how to make scientific results relevant to public policy more rapidly available. Expedited review, open access to journals during the epidemic, and the launch of sites such as biorxiv, in which authors could preprint publications prior to peer review, significantly changed the picture of how science can shape policy when it is needed the most. While publication of scientific papers prior to peer review could in some instances lead to the spread of false information, not providing timely science results in policymakers operating without the benefit of scientific input. Thus, a balance needs to be sought between defensible and credible science on the one hand and timely science on the other. On the issue at hand, if evidence arises that climate is important, it needs to be rapidly incorporated into models in order to feed into policy decisions.

Issues to Consider for Countries in the Northern Hemisphere

When considering the effect of climate on COVID-19, several issues should be top of mind to both policymakers and the general population.

 

The transition to spring and summer could help in controlling the present outbreak, but fall and winter weather will soon follow

First, while the 5–11°C range could be the optimal habitability range for this virus, environmental conditions are not the only factor enabling or limiting the spread of SARS-CoV-2. If travel restrictions or bans on social interaction are lifted too early, there is the risk of a second surge in the outbreak, especially in regions where typical warmer and moisture-laden conditions have not yet arrived. Second, and possibly more important, virulent strains such as SARS-CoV-2 carry the ability to rapidly evolve. If we place too much emphasis on waiting for nature to usher in the limiting conditions that will curb its spread, the particular category of virus we are fighting could evolve to become even more pathogenic than the current strain.

Finally, the transition to spring and summer could help in controlling the present outbreak, but fall and winter weather will soon follow in the Northern Hemisphere, providing the runway for a second wave of infections.

For Tropical Countries and Countries in the Global South

The language of “ticking time bombs” has been used in the media as a way of illustrating how immediate action in countries in the Global South can stave off catastrophe. Yet it is not entirely known why many of these bombs have not already exploded as they have in many countries in the Northern Hemisphere. There are many variables, considerable noise, and potentially a tremendous number of unreported cases. Yet one possibility is that COVID-19 will not show the same transmission patterns in tropical regions as it does in temperate ones. If transmission of COVID-19 follows the pattern of the flu, then the peak outbreak in tropical regions will occur during humid-rainy conditions that will arrive in Asia with the monsoon season. If this is true, it is important to spot it early on for a number of reasons.

Policymakers in tropical regions could start feeling like COVID-19 missed them and become complacent right before the epidemic hits

Policymakers in the Global South have constraints on enacting broad national lockdowns. The poor lack the resources to comply with extended shelter-in-place policies. Unlike in high-income countries with a higher percentage of workers in the knowledge economy who can telecommute to work, lockdowns could particularly devastate low-income countries. Locking down during times of the year when natural climate conditions are limiting transmission could use up economic and political capital before it is most needed. Policymakers in tropical regions could start feeling like COVID-19 missed them and become complacent right before the epidemic hits. A COVID-19 peak in tropical countries during the rainy season would coincide with the peaks of other vector-borne illnesses such as malaria and dengue. This would not only overwhelm health clinics, but there would be potential for coinfection and transmission both in the community and in the hospitals and clinics. This could be particularly devastating without proper planning as monsoon season does more than bring vector-borne illnesses. It can cause other disruptions, including the ability to transport equipment on flooded roads.

Because the timing of monsoon season is different across and within countries in Asia, countries should be vigilant in looking for outbreaks in regions that first receive the monsoon. In countries like India where monsoon season varies across the country, it should be possible to use random sampling approaches to effectively control for variations that can make climate effects difficult to isolate out in cross-country comparisons.

Application of the Precautionary Principle Yields Different Conclusions

The National Academy of Sciences’ April 7 assessment of the relationship between climate and transmission of COVID-19 gets to the heart of the problem for policymakers looking for scientific guidance—for at this point, scientists are on the fence. “In summary, although experimental studies show a relationship between higher temperatures and humidity levels, and reduced survival of SARS-CoV-2 in the laboratory,” the report said, “there are many other factors besides environmental temperature, humidity, and survival of the virus outside of the host, that influence and determine transmission rates among humans in the ‘real world.’”

Another way of stating this conclusion is, “We really don’t know.” In the absence of clear scientific guidance, perhaps the most appropriate action is to follow the precautionary principle, in which policy actions are weighed in terms of the consequences of being wrong.

The worst-case scenario in temperate high-income countries in the Northern Hemisphere is no link between transmission and climate. It would be disastrous after all to relax social distancing and other policies on the assumption that the summer will solve the problem only to find out that transmission is not reduced by increased heat and humidity in those countries. Better to assume no link between transmission and climate, keep social distancing in place, and be happily proven wrong as the summer months lead to a sharp reduction in cases.

In low-income tropical countries, the worst-case scenario would be if there is a link between transmission and weather

Yet in tropical countries, particularly low-income ones, it is far from clear that the same logic holds. For these countries, the worst-case scenario would be if there is a link between transmission and weather. If COVID-19 follows the transmission patterns of the flu in tropical countries, transmission will peak during the rainy season. This would be catastrophic, as COVID-19 transmission would peak at the same time as other vector-borne diseases such as malaria and dengue. Because health facilities are already overwhelmed in these countries in the rainy season by vector-borne diseases, there would be no surge capacity of health facilities to respond. It would also mean coinfection of COVID-19 and other vector-borne diseases, which would undoubtedly increase the severity of cases.

Thus, although it is prudent to assume a worst-case scenario situation in thinking about the climate-transmission link given the absence of evidence, the assumption regarding a climate link could be different in tropical and temperate countries. For low-income countries in tropical regions it could be particularly problematic to impose unsustainable national shutdowns during periods when the climate is naturally suppressing infection. This would be a double miscalculation if the ability to implement future national lockdowns is diminished right as transmission surges with the onset of the rainy season. 

The ramifications of COVID-19 for countries around the world are unprecedented. The scale of the disease itself is staggering and the side effects of policy efforts to curtail it also carry significant known and unanticipated side effects.

Right now the presumption is that there no climate effect on transmission, but this could be wrong

Given that public policy cannot count on the emergence of a silver bullet in the near term, countries will have to move between periods of system-wide lockdown and periods of opening up and using data and surveillance to suppress transmission. Although assuming a strong seasonal effect will solve the problem if there is no relationship between climate and transmission could be catastrophic, emulating northern countries’ approaches and timing could also result in profound social harm—particularly in the Global South—if transmission follows seasonal variations and different seasonal patterns in temperate and tropical regions. Right now, the presumption is that there is no climate effect on transmission, but this could be wrong. Policymakers need to react quickly if the preponderance of evidence begins to suggest otherwise. The effect of climate on transmission therefore is not a question we can afford to answer incorrectly, but neither is it a question we can stand to ignore.