# 5180
A timely study this morning, published in BMC Public Health, that looks at and attempts to quantify the impacts of mass gatherings and holiday travel on the spread of an influenza epidemic.
With the UK poised on the brink of their epidemic threshold this holiday season, we may well see this effect at work over the next several weeks.
Of course, the notion that influenza epidemics can surge in the wake of large gatherings of people is hardly new. During the 1918 Pandemic, most cities instituted strict public health ordinances. They closed schools, movie houses, pool rooms, restaurants . . even churches.
Those cities that took these measures generally saw much lower levels of death and illness.
But Philadelphia went ahead with a massive Liberty Loan parade on September 28th of that year, apparently heartened by the low number of flu cases reported in Pennsylvania to that point.
Days after Philadelphia’s Liberty Loan parade in September 1918, which was attended by 200,000 people, hundreds of cases of influenza were reported. [Credit: Naval Historical Center]
What happened next is best demonstrated by the following graph, which depicts an explosion of pandemic flu cases beginning just days after the parade.
The chart above, taken from the PNAS journal article entitled Public Health Interventions and Pandemic Intensity During the 1918 Influenza Pandemic , shows the excess mortality in two American cities.
The tall spike represents Philadelphia, while the lower curve represents St. Louis.
Over the next three weeks, at least 6,081 deaths from influenza and 2,651 deaths from pneumonia were registered in Pennsylvania, most occurring in Philadelphia (CDC source).
A more recent example comes from the 2009 H1N1 pandemic, which appears to have been efficiently disseminated around the world due to the number of Spring Break vacationers returning from Mexico during the initial outbreak.
But that was then, this is now.
With the exception of the UK (and possibly parts of eastern Europe), influenza appears to be simmering at fairly low levels in most parts of the northern Hemisphere.
The next eight weeks, however, will see a number of large public gatherings (i.e. Super Bowl, Carnival in Rio) and the busiest travel holiday of the year – Chinese New Years.
Any of which have the potential to spark a resurgence of the flu. Which brings us to the BMC Journal study, called:
The impact of mass gatherings and holiday traveling on the course of an influenza pandemic: a computational model
Pengyi Shi, Pinar Keskinocak , Julie L Swann and Bruce Y Lee
BMC Public Health 2010, 10:778doi:10.1186/1471-2458-10-778
Published:21 December 2010
Abstract (provisional)
Background
During the 2009 H1N1 influenza pandemic, concerns arose about the potential negative effects of mass public gatherings and travel on the course of the pandemic. Better understanding the potential effects of temporal changes in social mixing patterns could help public officials determine if and when to cancel large public gatherings or enforce regional travel restrictions, advisories, or surveillance during an epidemic.
Methods
We develop a computer simulation model using detailed data from the state of Georgia to explore how various changes in social mixing and contact patterns, representing mass gatherings and holiday traveling, may affect the course of an influenza pandemic. Various scenarios with different combinations of the length of the mass gatherings or traveling period (range: 0.5 to 5 days), the proportion of the population attending the mass gathering events or on travel (range: 1% to 50%), and the initial reproduction numbers R0 (1.3, 1.5, 1.8) are explored.
Results
Mass gatherings that occur within 10 days before the epidemic peak can result in as high as a 10% relative increase in the peak prevalence and the total attack rate, and may have even worse impacts on local communities and travelers' families. Holiday traveling can lead to a second epidemic peak under certain scenarios.
Conversely, mass traveling or gatherings may have little effect when occurring much earlier or later than the epidemic peak, e.g., more than 40 days earlier or 20 days later than the peak when the initial R0 = 1.5.
Conclusions
Our results suggest that monitoring, postponing, or cancelling large public gatherings may be warranted close to the epidemic peak but not earlier or later during the epidemic. Influenza activity should also be closely monitored for a potential second peak if holiday traveling occurs when prevalence is high.
The complete article is available as a provisional PDF. The fully formatted PDF and HTML versions are in production.
Of course, as Professor Emeritus of Statistics at the University of Wisconsin George E. P. Box famously observed:
“All models are wrong, but some models are useful.”
And the authors of this study readily admit that there are limitations to computational studies such as this. They write:
Computer simulations by definition are simplifications of real life. Rather than make decisions, they can identify potentially important factors and relationships for decision makers.
Our model does incorporate a number of assumptions and cannot fully capture every possible factor or effect.
Each year, epidemiologists watch Chunyun, or the Spring Festival Travel Season (of which Chinese New Years is a central part) with particular fascination.
It is, quite rightfully, billed as the largest annual migration of humans on the planet. Chunyun begins about 15 days before the Lunar New Year and runs for about 40 days total.
This year, the Lunar New year falls on February 3rd, and with it comes a cultural ethic for millions who have moved to the big cities to return home to visit with their families.
These travelers will spend a few days in their home village or town, and then travel back to the city where they work or attend school. Which can provide ample opportunities for an infectious disease to hitch a ride.
Of course, it should be pointed out that last year, Chunyun did not precipitate a major resurgence in the H1N1 pandemic virus across Asia.
Nor did we see any feared third wave of influenza following Carnival in Rio, the Super Bowl, or the the World Cup in South Africa.
And while there are yearly concerns about bird flu being brought back to the cities from rural locations by Chunyun travelers, that doesn’t appear to have happened (yet) either.
All of which means that just because the potential exists, that doesn’t guarantee that something bad will happen.
Of course, the fact that it didn’t happen last year doesn’t preclude it from happening this year . . . or in the future.
So we’ll watch for signs that family gatherings and travel during this Holiday season may have exacerbated the the flu epidemic in the UK. And we’ll keep a watchful eye on influenza activity and spread in the wake of this year’s Super Bowl, Carnival, and Chunyun.
With luck, and over time, with the data we get from computer simulations like the one above - and from observations from the real-life laboratory of the world - we’ll learn a lot more about how, why, and under what conditions influenza is most likely to spread.
Valuable information during any influenza season, but of particularly importance should we ever face another virulent pandemic.
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