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expert reaction to modelling study looking at risk of international Zika virus spread represented by 2016 Olympics in Brazil

A group of researchers publishing in the journal Annals of Internal Medicine have used a modelling study to estimate the risk of Zika infection to those traveling to the Olympics in Brazil and suggest the number of infected travelers returning to all countries to be between 3 and 37.

All our previous output on this subject can be seen here. The SMC also produced a Factsheet on the Zika virus.


Prof. Mike Turner, Director of Science and Head of Infection and Immunobiology, the Wellcome Trust, said:

“This study suggests that for spectators and athletes travelling to Rio this summer, the risk of contracting Zika virus is very low indeed. What risk there is can be mitigated further by taking measures to prevent mosquito bites, such as applying a high quality insect repellent regularly.

“The same cannot be said, unfortunately, for those living in regions of active Zika transmission, and we must step up the international research effort to understand the Zika virus to enable the development of effective interventions needed to protect those most at risk.”


Dr Nathalie MacDermott, Clinical Research Fellow, Imperial College London, said:

“The paper by Lewnard et al uses disease modelling to identify the risk to individual travellers to the Olympics of acquiring Zika virus infection, as well as the risk of infected travellers importing Zika virus to their home nations which have no known Zika virus transmission.

“The model confirms both the low risk of infection in individual travellers and the even lower risk of transmission of the virus to uninfected countries.  This confirms the WHO statement that it is safe for travellers to attend the Olympics, with the exception of pregnant women, and that the likelihood of transmission to and then within currently uninfected countries as a result of the Olympics is very low. The low risk as a result of the Olympics is further emphasised when taking into consideration the large number of travellers to the region annually for reasons other than the Olympics.”


Dr Derek Gatherer, Lecturer in the Division of Biomedical and Life Sciences, Lancaster University, said:

“Those who have argued for the postponement of the Rio Olympics due to the risk posed by the ongoing Zika virus epidemic base their argument on two principal assumptions: 1) that the half a million visitors to Brazil will generate a substantial increase in exported Zika infections, and 2) that some of these exports will seed outbreaks in other parts of the world.

“This paper seeks to calculate precise figures for these assumptions, using data on traveller numbers and the export of other mosquito-borne viruses from other major sporting events in Brazil – principally the export of dengue fever virus from the 2014 FIFA World Cup.  They conclude that the extra risk posed by Olympics-related travel is small in proportion to the existing risk.  The USA alone has already imported over one thousand Zika infections via routine travel to risk areas, and the extra travellers visiting Brazil for the Olympics will only represent an additional 100 or so, at most, exported infections to the world as a whole.

“The second assumption of the postponement lobby, that Africa is at particular risk of a microcephaly outbreak similar to that seen in Brazil, is also shown to be unlikely, as the authors calculate that only a handful of the predicted extra 100 exported infections are likely to go to Africa, which represents only 4% of the visiting population.

“The paper does not deal with the other weakness of the second assumption of the postponement lobby, which is that Zika is known to have been already circulating in Africa and, where tests have been carried out, levels of antibodies to Zika virus are present in African populations which would make subsequent spread of Zika unlikely.  This paper supports the sensible opinion that although Zika does constitute a global health risk, and Brazil is a site of ongoing transmission, holding the Olympic Games in Brazil will not substantially increase the risk to other parts of the world.”


Prof. Jimmy Whitworth, Professor of International Public Health, London School of Hygiene & Tropical Medicine, said:

“It is thought that the current Zika outbreak in Latin America may have been started by infected individuals from Pacific islands attending a sports event in Brazil. So it is perhaps appropriate that there should be concerns that attendees at the Olympics Games might disseminate Zika to other parts of the world – this paper provides low estimates of the chance of this happening, reinforcing and extending previous published work.

“Of particular interest, the authors estimate that attendees returning to African countries, which are thought particularly vulnerable to Zika outbreaks, would be infectious to mosquitos for a total of between 1 and 14 days. These are low numbers in comparison with the overall opportunities for the virus to spread internationally through travel.

“The authors provide reassurance that the Olympics will not significantly affect the risk of international spread of Zika, but with less than two weeks to go to the start of the Games, I can’t help thinking that this information would have been even more useful if it had been available weeks or months earlier.”


‘Low risk of international Zika virus spread due to the 2016 Olympics in Brazil’ by Joseph A. Lewnard et al. published in the Annals of Internal Medicine on Monday 25 July 2016. 



Declared interests

Dr Nathalie MacDermott: “I am a paediatrician. I am also undertaking a PhD in Ebola virus disease at Imperial College London funded by the Wellcome Trust.”

Dr Derek Gatherer: “Derek Gatherer has no conflicting interests with regard to Zika virus.”

Prof. Jimmy Whitworth: “Jimmy Whitworth directs the ERAES programme (Enhancing Research Activity in Epidemic Situations) at the London School of Hygiene & Tropical Medicine which is supported by the Wellcome Trust to provide funding for urgent research during outbreaks.”

No others received.

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