Figure 1. Social distancing during the holidays causes two, distinct peaks in influenza prevalence during an epidemic (dashed line), but travel causes the second peak to become larger (solid line). (Source, figure 3)
With the holidays only a few weeks away, now’s the time to finalize travel plans and stock up on cold and flu medicine. Each year, millions across the globe travel for the holidays. Travel surrounding Thanksgiving increases more than 50% from the rest of the year in the U.S., and for 2016, AAA estimated that 48.7 million U.S. citizens would take to the roadways and skyways–1 million more than last year. Comparatively fewer travel for Christmas and New Year’s, but traffic is still about 23% above average. But with the increase in human travel, what microbial gifts are we bringing home with us?
Well, the flu for one. The influenza virus causes a respiratory illness, spreads through droplets effectively dispersed by the symptomatic coughing and sneezing, and survives for days on doorknobs and handrails. Infected individuals can spread the virus a day before, and for up to a week after, the onset of symptoms. Per the U.S. Centers for Disease Control and Prevention, flu season extends from October to May, most often peaking from December to February—coinciding with colder weather, crowded classrooms, and the holidays. Do the holidays, and the massive increase in holiday travel, influence the flu season?
Inspired by the 2009 H1N1 influenza pandemic, a group at the Georgia Institute of Technology used computer modeling of influenza infections in Georgia to understand the impact of large gatherings and holiday travels on the spread of the virus. Their data indicated that the holidays created two prevalence peaks within an epidemic, that is, two distinct times when larger portions of a population have the flu (with or without symptoms). The authors ascribed the dual peaks to a phenomenon called “social-distancing”.
During the holidays, people change who they interact with both by visiting extended family and by interacting less with their everyday social groups (e.g., work, school, community soccer league). According to the authors, this briefly halts new cases of influenza until the end of the holidays when the social groups reconvene, leading to another rise in influenza prevalence. But how does this scenario work?
A 2016 preprint article identified one component of social-distancing causing the double peak: schools. More specifically, school holiday breaks. Researchers from Georgetown University and the National Institutes of Health used nationwide data on influenza-like illnesses from 2001 to 2009 to compare infection dynamics in adults and children over the holidays. Like the Georgia model, nationwide data showed a consistent decrease in (symptomatic) influenza prevalence over the holidays, causing the dual-peak phenomenon attributed to “social-distancing.” According to the model these authors made from the nationwide data, the second peak only occurs if school closures do also. They hypothesized that during school closures there are fewer interactions outside of family groups, reducing overall influenza transmission.
But school closures are only part of social-distancing. Remember that there are many different strains of the flu virus, that’s why we get a new flu vaccine each year. The social groups that people stick to throughout the year at work or school pass around certain strains of influenza. After the strain passes through the group it creates a small pocket of herd immunity against that strain, but not necessarily others. Interaction with other social groups (like through extended family members, festivals) results in the introduction of new influenza strains after return to work/school after the holidays. Now there’s spread of the newest flu strain through the workplace, resulting in a second bout of illness and prevalence peak (about 5 weeks into the new year according to the nationwide data).
Figure 2. Photo by: Quinn Dombrowski (Source)
While social-distancing is definitely the cause of the second peak, holiday travel changes how high that second peak climbs. If only 25% of a community travels at least 50 miles away from home, the second peak in influenza prevalence increases. In other words, transmission of the flu decreases during holiday breaks and picks up again in the new year regardless of weather people travel. But if people travel, more people get influenza during the second prevalence peak than if everyone had just stayed home.
The timing of travel and holiday breaks also has an impact on influenza prevalence. The Georgia model indicated that if travel and large gatherings (like festivals or religious services) occur just before an epidemic peaks, then influenza prevalence is amplified to a degree that could strain health providers and resources in multiple communities simultaneously.
The duration of a mass gathering is important too, since longer events are less likely to negatively impact an epidemic. For instance, people attending a one- or two- day event will mingle and spread influenza among themselves before leaving to another event or community, carrying their viral hitchhikers along with them. At a longer gathering, however, exposed attendees stay long enough to get sick, and possibly get better, restricting the spread of the virus outside of the event.
In a nutshell, school holiday breaks and pockets of strain resistance in social groups cause two peaks in an influenza epidemic while holiday travel and large gatherings serve as influenza swap meets. What can you do to stay healthy over (and after) the holidays? Check out the CDC’s “Twelve ways to health”—suggestions that can be sung to “The twelve days of Christmas” during your lengthy car ride—and get your flu shot. Getting vaccinated today (Friday, Dec. 9th) means that you’ll have anti-influenza antibodies in your stocking (er, bloodstream) come Christmas morn.