In a startling turn of events, Paris, France, has experienced an unprecedented outbreak of dengue fever, marking the northernmost occurrence of the mosquito-borne disease on record. What makes this outbreak even more alarming is the fact that none of the affected individuals had recently traveled, indicating the possibility of local transmission in northern Europe. This development raises concerns for the upcoming Paris Olympics, which is expected to draw over 10 million athletes, spectators, officials, and tourists to the city in 2024.

Realizing the inherent risk of dengue, the French government has taken preventive measures by conducting regular checks for the presence of dengue-carrying mosquitoes across hundreds of sites in Paris. However, the question remains whether these efforts will be enough to prevent a potential outbreak during the international event.

The concept of a "super-spreader" in the context of infectious diseases is not new. It refers to a small fraction of the population, often just one individual, being responsible for the majority of the cases. One infamous historical example is "typhoid Mary," a carrier of typhoid fever who unknowingly infected over 100 people.

Recently published research in the journal Nature suggests that 15% of people in Hunan Province, China, were responsible for 85% of the COVID-19 cases. Similarly, an analysis conducted in Peru regarding dengue revealed that approximately 8% of human-occupied spaces accounted for over half of the cases. It is important to note that dengue cannot be transmitted directly from one human to another; it requires the bite of a dengue-carrying mosquito.

The correlation between large-scale international events and viral epidemics is not unprecedented. During the 2016 Olympics in Brazil, concerns about the Zika virus, another disease transmitted by the Aedes mosquito, almost led to the postponement of the games. Similarly, the fear of COVID-19 transmission during the Tokyo Olympics resulted in stringent measures to limit the spread of the virus. While infections within the Olympic bubble were low, there was an increase in cases among the general population.

Since 2016, the range of the Aedes mosquito has expanded significantly, and dengue cases worldwide have witnessed a dramatic surge. In 2016, there were 5.2 million reported cases globally, while midway through 2024, the number has already reached 7.6 million.

With visitors expected from over 200 countries, many of which are already experiencing dengue outbreaks, the Paris Olympics pose an even higher risk. For the event to become a super-spreader, several factors must align, including the presence of enough mosquitoes, a sufficient number of susceptible and infected individuals, ample time, and a significant number of mosquito bites.

The tiger mosquito, well adapted to the urban environment in Paris, requires only a tiny amount of water in small containers to lay its eggs. It predominantly feeds on humans during dawn and dusk, and its eggs can withstand dry conditions for months, hatching when exposed to water again. What makes this situation particularly dangerous for Paris is that some of these mosquitoes may already carry the dengue virus, passed down from their mother, potentially reducing the number of bites required to initiate an epidemic.

Within the timeframe of the Olympics, a single bite from an infected mosquito could spark an epidemic within a week. Each female mosquito is capable of laying up to 200 eggs at once. It is crucial to note that most dengue cases are asymptomatic, meaning individuals infected before or during the Olympics may unknowingly carry the virus and inadvertently trigger an epidemic upon returning to their home countries.

Whether individuals fall ill or not, they can still transmit the virus if bitten by an Aedes mosquito. In the case of the Rio Carnival earlier this year, a dengue outbreak occurring just days before the event led to a public health emergency declaration, but the carnival proceeded as scheduled.

Paris, however, faces a unique challenge as the Olympics itself becomes a risk factor. Everyone present in the city during the Olympic period, including residents, workers, visitors, participants, volunteers, and even passersby, will inadvertently become part of a substantial natural experiment, unaware of their potential role in the transmission of dengue.

As the Paris Olympics approach, authorities must remain vigilant and implement thorough and rigorous measures to mitigate the risk of a dengue outbreak. The health and well-being of not only the Olympic participants and attendees but also the global population depend on the effectiveness of these preventive measures to ensure a safe and secure event.