Risk of infection by Zika virus during Rio Olympics divides experts

Letter sent to The Lancet by researchers at the University of São Paulo and Singapore's Lee Kong School of Medicine contests call to postpone or move 2016 Summer Olympics addressed to WHO by 150 scientists.

By Karina Toledo  |  Agência FAPESP – “If you are not pregnant and decide not to attend the Rio Olympics this year out of fear of Zika infection, you'd rather find a better reason; there are many others.”

This statement is by researchers Eduardo Massad, Francisco Coutinho – both affiliated with the University of São Paulo’s Medical School (FM-USP) – and Annelies Wilder-Smith from Singapore’s Lee Kong School of Medicine. All three are signatories of a letter submitted on May 31 to The Lancet. Massad is a member of the Zika Virus Research Network in São Paulo (Rede Zika), supported by FAPESP.

In the text, which has yet to be approved for publication by the journal, the experts in mathematical modeling and epidemiology compare the risk of infection by Zika virus during the Olympics, scheduled to take place in August, with the risk of acquiring dengue fever, considered low during the drier, cooler months of this period, which is winter in southeastern Brazil (read more at: http://agencia.fapesp.br/23285/).

The authors set out to respond to an open letter addressed to World Health Organization (WHO) Director General Margaret Chan calling for the 2016 Summer Olympics to be postponed or moved elsewhere. The document was signed by 150 doctors, public health specialists and bioethicists from several countries, including Débora Diniz, a Brazilian who teaches at the University of Brasília (UnB).

According to the signatories of the letter sent to the WHO, holding the Games in Rio risks accelerating the spread of the Brazilian viral strain around the world. They also argue that it is unethical to run such a risk in light of new scientific findings that underscore the seriousness of the problem.

For Massad, however, the open letter is not backed by scientific evidence. “We calculated the individual risk of infection by dengue virus for foreign visitors to the Rio Olympics, which is 0.0005, i.e. 5 cases per 10,000 visitors. The individual risk of infection by Zika is some 15 times less, or 0.00003 – roughly 3 cases per 100,000 visitors. If the number of foreign visitors expected is about 500,000, that means about 15 could be infected – ten cases would be asymptomatic and five symptomatic,” Massad told Agência FAPESP.

According to Massad, the calculation was based on the estimated number of Zika cases in Brazil in 2015, which was between 500,000 and 1.5 million.

“Notification is unreliable, so we estimated the number of cases indirectly,” he said. “For example, we know that for every clinical case of Zika there are five asymptomatic cases. We also know that 1 in 100 pregnant women infected by Zika will deliver a baby with microcephaly. Because the number of babies born with microcephaly due to Zika is known, we can calculate the number of infected pregnant women. And because we know the ratio of pregnant women infected with dengue to the total number of dengue cases, we can use the same ratio to calculate the total number of Zika cases.”

The number of dengue cases notified in 2015 surpassed 1.6 million, according to a report issued by Brazil’s Ministry of Health early this year. Many asymptomatic and symptomatic cases were evidently not notified and should therefore be added to the official number.

“If the distribution of Zika cases from month to month is the same as that of dengue – and there’s no reason why it should be different, as both viruses are transmitted by the same mosquito and it’s the vector that determines seasonality – we can predict that the risk of infection by Zika in August is 15 times less than the risk of infection by dengue,” Massad said.

In the text sent to The Lancet, the scientists say the risk of being raped in Rio for women, or of being shot to death for men, is about ten times greater than the risk of infection by Zika.

“Of course, these are very crude estimates,” Massad said. “Although the risk is very small, we too advise pregnant women not to come to the Games. But the call to delay or move the Games isn’t justified.”

A different opinion

Asked for her reaction by Agência FAPESP, Diniz said she was “astonished” by the confidence with which the text submitted to The Lancet presents numbers to predict the risk of infection by a virus about which so little is known. “We don’t even know the attack rate for Zika, for example, meaning the speed of spread in the at-risk population in a short period of time,” she stressed. “Notification only became mandatory in January 2016 in Brazil, so we don’t know much about the history of the disease either, but this is crucial when you’re predicting a future risk. In sum, these numbers must be accompanied by a warning about their uncertainty.”

According to Diniz, not just expectant mothers are at risk but all women of reproductive age, as well as men who have sex with these women and their joint reproductive plans. 

“The WHO’s latest recommendation, for example, is that men with symptoms of Zika should wait six months before planning a pregnancy with a partner,” she said. “What we’re talking about here is the reproduction of a population. Unfortunately, it seems to us that there are several good reasons for deciding to avoid the Rio Olympics.”