Winter Is Coming: Modeling Seasonality Of Zika Virus


Pathogens, brace yourselves. The winter is coming effect can prematurely end your outbreak, even if conditions initially seem to be in your favor.

A new study released this week on bioRxiv identified a new seasonality effect—the winter is coming effect.

Understanding the winter is coming effect could help eradicate disease more quickly. Pulse vaccinations are a strategy to eradicate disease, which involves repeatedly vaccinating people in a defined age range until the disease is stopped. Knowing the most effective time to use pulse vaccinations will help with allocating the vaccines to the places that need it the most, at the right time.

In order to effectively respond to outbreaks, researchers focus on understanding early warning signs. Classic epidemiological models used to predict outbreaks depend on factors like transmission rates, clusters of susceptible people, and recovery versus mortality rates of people who get sick.

The classic model generates a single number. If it’s above 1, the disease will spread. If it’s less than 1, the outbreak peters out.

This model is highly simplified and doesn’t account for changes in transmission and susceptibility through time. Shifts in environmental conditions, like temperature and humidity, can change how easily pathogens infect people. The environmental effects are magnified if a disease relies on a vector, like mosquitos or ticks, to infect more people.

The researchers, based in France, used a model that captures the complexity of a vector-borne disease. Researchers discovered that even when pathogen transmission rates are high, the likelihood that there will be an outbreak can be “vanishingly small”. To understand this discrepancy, they focused on exploring variation explained by seasonality.

According to the researchers, “when the introduction time of the pathogen is shortly followed by a low transmission season, the introduced pathogen is doomed because it will suffer from the bad times ahead. We call this the winter is coming effect.”

When the researchers used a simple model, they found that the approximation didn’t capture the winter is coming effect. So they used a new theoretical framework to compare different public health strategies to prevent outbreaks.

The new model parameters were based off of Zika virus transmission, which spreads through mosquitos. Seasonality alters how dense mosquitos are in the area, which influences how fast and how far Zika virus can reach.

Even if other parameters are in favor of the virus spreading (e.g., high transmission rates or low recovery rates of those infected), the model shows that a Zika outbreak isn’t likely to happen if the period is followed by extremely tough conditions. The virus would have to get a significant headstart to beat unfavorable environmental conditions.

Finally, the researchers found if eradicating the disease isn’t possible, the next best strategy is to focus intensive efforts in a short period of time. Completing pulse vaccinations almost four months in advance of peak transmission period is more effective than the three months ahead, which is the current strategy. This simple change in the timing of pulse vaccinations could help reduce Zika outbreaks.

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