A mass gathering on the scale of the Olympic and Paralympic Games inevitably focuses international scrutiny on the host city’s ability to deliver a successful and safe event. This scrutiny extends to the host’s public health systems. Large international mass gatherings have the potential to encourage the spread of infectious diseases and, although the potential has not often been realized in recent times, it warrants effective preparation to detect and respond to any possible threats. At the same time, mass gatherings can be an opportunity to develop an enhanced public health system as a legacy for the host country.

The challenge, in the seven years between being awarded the Games and delivering them, is to assess the likely risks that might arise, identify any gaps in existing public health systems and enhance the systems to fill the gaps. Every host city will deliver their Games in their own way, both because they need to work within their normal systems and culture and because the risks and challenges can be different in different places.

There is, however, a process for sharing learning from one host city to subsequent hosts, supported by the World Health Organization and a global network of WHO Collaborating Centers for Mass Gatherings (including Public Health England). As part of this process, London hosted a “Health Observer Programme” during the Games involving future host cites including Rio de Janeiro. There have also been exchange visits between London and Rio as well as multilateral discussions between those cities, WHO and the International Olympic Committee to share lessons that could be incorporated as necessary into Rio’s planning.

As Olympic host cities, London and Rio have faced and will face many similar issues, but also many different ones. Both are major cities that regularly host large-scale events and both manage millions of international visitors every year. In health terms, however, London has effectively no insect vectors to transmit diseases whereas Rio had to consider diseases like dengue and chikungunya as threats even before Zika emerged. The U.K. has a unified national health system; Brazil has a multilevel system with health responsibilities at local, regional and national levels, so coordination may be more challenging. The U.K.’s health service is predominantly state-funded and publicly provided; Brazil has a more mixed economy of public and private provision. All of these mean that Rio’s Games will be run in a Rio way, not as an emulation of London (or any previous Games).

Rio—as did London—will focus on those things that are most likely to happen during the Games: gastroenteritis and food poisoning, respiratory illnesses, small clusters of vaccine-preventable diseases such as measles or chickenpox, and sexually transmitted infections. These are things that happen all the time in cities; they may or may not be more common during the Games but they will happen and will need a response. Water-borne diseases will also be of concern because of considerable media attention on water quality at the rowing, sailing and swimming venues. Remediation efforts by the Rio authorities have now reduced the risk but it will still be on the minds of the public health authorities.

Evidence on mass gatherings, however, suggests that the real “Olympic difference” is not a rise in disease events but rather the scale of interest—both political and media—that is focused on anything that happens during the Games (even if unrelated to them). The large number of media teams present means any event can become a big story quickly, and the political importance of delivering a “good” Olympics means the risk tolerance of politicians is very low.

Zika may be a good example of this. Although the consensus across international public health organizations is that Zika will not be a major issue in Rio because the mosquito population drops during Southern Hemisphere’s winter, political and media interest in Zika is nonetheless very high. There will be many people looking for, and perhaps even hoping for, Zika cases linked to the Rio Games. That will add more pressure to the public health system, ensuring that the very intensive Zika measures Brazil has developed over the last nine months will continue throughout their winter.

For London, this sort of attention focused on two specific factors: surveillance systems’ ability to detect something quickly and the capability to reassure the public and politicians that nothing bad was happening.

In general, surveillance systems are not designed to work at the speed that is expected in the Olympic context, and they are not designed for reassurance. In order to do that you need a paradigm shift in surveillance, moving from systems that tell you when something happens to systems that can reliably tell you that nothing is happening.

The response in London was twofold: enhance existing systems and look to get additional information as well as establish an efficient means to assimilate the information.

A major focus was on syndromic surveillance. This uses the symptoms people complain about (diarrhea, vomiting, sore throat, etcetera) rather than what doctors diagnose. Because it doesn’t rely on confirmed diagnoses, it can produce information quicker than conventional systems—although that information is less specific.

The U.K. has had well-developed syndromic systems for many years based on both primary care and nurse-led telephone helplines. For 2012 we added new hospital emergency department (ED) systems that extracted information from routine ED information systems. This gave two advantages: It enhanced the range of information available and allowed us to triangulate across three systems (laboratory, clinician reporting and syndromic). Triangulation allowed us to have more confidence that nothing was happening when all three systems were “quiet”.

The situation in Rio will be different because Brazil does not have the same history of syndromic surveillance systems and it is not easy to develop these from scratch as part of Olympic planning. Rio will therefore focus more on conventional disease surveillance for its information.

To ensure we could deliver information effectively in London, an Olympic Coordination Center (OCC) was created. From early 2012 we started running the systems in “Games Mode,” initially every month, then weekly and moving to daily from June to October. From July the OCC was operational seven days a week; physically open for 16 hours a day and virtually for eight hours overnight.

The OCC was essential as the single point where all information came together—from national surveillance and laboratory systems, the syndromic surveillance team, local public health teams around the country, media reporting and epidemic intelligence internationally coming from WHO and the European Center for Disease Prevention and Control. Each team collated their contribution and e-mailed it to the OCC for a daily review meeting. This allowed us to generate a “Situation Report” (SitRep) that represented the agreed single version of the truth across the public health system. The OCC also served as the single point of contact for other agencies to ask questions. The SitRep was sent to the government coordination center and to the Games organizers at 6 A.M. each day.

Similar coordination systems will be active in Rio to link the health system, the government coordination system and the Games coordination system. That system may be more complex than London’s because of Brazil’s multitiered government.

An important aspect of this is coordination between the health and security sectors. This is often not well developed but is essential in mass gathering planning because the interactions between the sectors if there is a major incident (whether naturally occurring or deliberately caused) will be critical, and therefore building trust and collaboration in the planning stages is vital. This was one of the key learning points emphasized from London.

So Rio’s Games will be Rio’s. They will prepare for and respond to different challenges than those faced by previous host cities but they will do it within an expanding global framework for mass gathering planning. Rio will contribute their unique perspective to that framework in due course—after their closing party!