How Mobility Influences Respiratory Virus Spread

In early 2020, many countries started using health measures to slow down the spread of COVID-19. These measures included staying at home, limiting gatherings, closing schools and businesses, and banning travel. These actions helped not only reduce COVID-19 cases but also kept other Respiratory Viruses from spreading as much. Many of these viruses did not show up again until late 2020 or 2021, which happened after people started to socialize more again.

During the pandemic, data from mobile phones became crucial in tracking how people moved and how this affected the spread of COVID-19. Researchers used this information to model how COVID-19 spread and to evaluate the effectiveness of the health measures. However, not many studies looked into how changes in people's movements affected the spread of other respiratory viruses during this time. We define "Endemic" respiratory viruses as those that regularly circulate in the population and have stable rates of infection.

At the start of the pandemic, people were not exposed much to these endemic viruses. This lack of exposure meant that many people became more susceptible to them, potentially leading to larger outbreaks later on. Understanding how people's movements relate to the spread of these viruses is key to predicting future outbreaks, especially because simultaneous outbreaks from different viruses can overwhelm healthcare systems.

In this study, we use detailed health data and mobile location data to look at the connections between how people behaved and how 17 common respiratory viruses spread in the Seattle area from 2018 to 2022. These viruses include COVID-19, various types of influenza, respiratory syncytial virus (RSV), and several coronaviruses.

#Study Overview

This research uses data from the Seattle Flu Study (SFS), initiated in late 2018, to better recognize and control epidemic and pandemic situations. The SFS gathered extensive health data by systematically testing nasal swabs for many respiratory viruses in both hospitals and the community. The study covers data collected from November 2018 to June 2022, which includes samples from sick and healthy individuals across the Seattle area.

In total, 138,060 samples were screened for up to 26 viruses, and we focused on 80,846 samples, which came from individuals showing symptoms. Out of these, a significant portion was collected from hospitals and most were from community-based testing sites, such as clinics and testing kiosks.

Over four years, about 40.6% of the samples tested positive for at least one respiratory virus. Before COVID-19 health measures began in March 2020, the most common pathogens were various strains of influenza. After those measures began, COVID-19 became the most prevalent virus detected.

We created daily counts for COVID-19 and other endemic viruses, taking into account how many tests were done, the age of the individuals, where they were tested, and how many people were reporting respiratory illnesses. Even though our study tested for many viruses, we concentrated on 17 viruses that had enough positive samples during the study period.

#Mobility and Behavioral Trends

To investigate how changes in people's behavior and movements influenced the spread of these viruses, we used mobile device data alongside our health surveillance data. This data included information on how often people visited different places, such as schools, restaurants, and healthcare facilities.

We focused on key events in Seattle, such as a significant snowstorm in February 2019, the State of Emergency declaration for COVID-19 in late February 2020, and the stay-at-home orders starting in March 2020.

The major snowstorm in February 2019 resulted in many closures and limited movement. This drastic change in human behavior led to noticeable drops in the circulation of several respiratory viruses. After the storm, we saw significant reductions in the Transmission of viruses like RSV and adenovirus, as fewer people were moving around and gathering in public spaces.

When COVID-19 restrictions came into effect, there was an immediate and substantial drop in mobility. Movement to places like restaurants and schools dropped significantly, and people began staying home much more. This led to a noticeable decline in the transmission rates of various respiratory pathogens.

As restrictions eased and people began to move about more freely, we observed rebounds in certain viruses. The spread of hRV and AdV began to rise rapidly again in June 2020, right after many businesses reopened. This quick rebound indicates how closely linked mobility is to virus transmission.

#Impact of COVID-19 Restrictions

The arrival of COVID-19 changed the way we understood the spread of respiratory viruses. As the first wave of COVID-19 happened, public health measures brought about a significant decline in cases for all respiratory viruses. This decline had not been seen by the end of February 2020.

In March, as restrictions intensified, the transmission rates for endemic viruses significantly dropped. Moreover, the lockdown measures had a long-lasting effect on how viruses circulated. Even as restrictions were lifted, the resurgence of endemic viruses was not uniform. Some viruses began circulating again more quickly than others, which was influenced by human behavior.

Schools began remote learning, reducing contact rates among children and affecting how viruses like influenza and RSV returned to circulation. The long absence of these viruses led to a naïve population that was more susceptible when they returned, allowing for more significant outbreaks than would typically be expected.

#Surveillance Data and Key Findings

Our study combined various data sources to analyze the behavior of respiratory viruses in relation to human mobility. Through ongoing surveillance and detailed analysis, we were able to observe the dynamics of multiple viruses during the pandemic period.

Key findings include:

1. Mobility Predicts Virus Transmission: We observed that changes in human mobility were closely associated with the rise and fall of respiratory virus transmission. High foot traffic in schools, childcare centers, and other public spaces was frequently linked to increased transmission rates.

2. Impact of Snowstorm: The snowstorm in February 2019 saw significant declines in virus transmission, showing how sudden changes in behavior could have immediate effects on the spread of illness.

3. COVID-19 Health Measures: Restrictions for COVID-19 led to a noticeable decline in the spread of other respiratory viruses, demonstrating that social distancing and other measures can effectively reduce overall transmission.

4. Delayed Resurgence: The rebound of certain viruses occurred much later than expected, possibly due to people being less exposed or due to broader behavioral changes, such as reduced travel and gatherings.

5. Resurgence Patterns: Non-enveloped viruses, like hRV and AdV, showed quicker rebounds compared to enveloped viruses, likely due to their ability to survive longer in the environment and persist in the population.

6. Behavioral Changes During Pandemic Waves: During different COVID-19 waves, the relationships between mobility and the transmission of SARS-CoV-2 differed based on the virus's characteristics and the public's behavior in response to those characteristics.

7. Immunity Debt: The prolonged lack of exposure to typical endemic viruses may have led to a situation where many individuals had weakened immunity, resulting in larger outbreaks when these viruses returned.

8. Future Implications: As SARS-CoV-2 continues to become more endemic, the dynamics of mobility and behavior may shift similarly to those we see with other established respiratory viruses.

#Conclusion

The relationship between mobility and the dynamics of respiratory viruses is crucial to understanding how outbreaks occur and how to manage public health responses. Our study demonstrated that people's movements and behavior play a significant role in virus transmission, particularly during periods of dramatic change.

As we look ahead, it is essential to monitor mobility trends and their impacts on respiratory viruses, especially as societies continue to navigate living with COVID-19 and other endemic viruses. By using mobile device data alongside traditional health surveillance methods, we can gain valuable insights into how to mitigate future outbreaks and protect public health.