Will the Zika infection rate during Olympics be lower?

Zika case in Rio between Jan and July 2015

The number of Zika virus cases confirmed (gray background) and not confirmed (white background) between January 1, 2015 and July 31, 2015 in Rio de Janeiro State. – Photo: PLOS Neglected Tropical Diseases

In an open letter to Dr. Margaret Chan, Director-General of  WHO, 150 scientists and doctors had cited several reasons why the Olympics, the world’s biggest sporting event to start from August 5, to has to be postponed and/or moved to another location.

But on May 28, the WHO clearly stated that “based on current assessment, cancelling or changing the location of the 2016 Olympics will not significantly alter the international spread of Zika virus”.

The WHO’s reasoning was that Zika virus transmission was ongoing in 60 countries, including Brazil.  And since no ban or restriction on travel to the any of the Zika-endemic countries was in place, the best way to reduce risk of disease is to follow public-health travel advice.  “We have at least 40 million visits from the U.S. to places around the world where Zika is spreading,” Dr. Tom Frieden, Director of CDC said while addressing the National Press Club on May 26.

“Travel to the Olympics would represent less than one quarter of 1 per cent of all travel to Zika-affected areas.  Even if you were to say the Olympics weren’t to happen, you would still be left with 99.75 per cent of the risk of Zika continuing to spread,” he said.

One of the most important points raised by scientists is that Rio is “highly affected by Zika” with the suspected cases being the “highest of any state in Brazil (26,000), and its Zika incidence rate is the fourth worst (157 per 100,000)”.  In other words, though located southeast and far away from the heart of the Zika outbreak in northeast, “Rio is not on the fringes of the outbreak, but inside its heart.”

To strengthen their argument, the scientists had pointed out that despite the massive programme to eliminate Aedes aegypti mosquitoes that transmit the virus to humans, mosquito-borne disease has only shot up.  Since no historical data on Zika cases exists, they used the incidence of dengue cases as a proxy to highlight the failure of the mosquito control programme.  From January to April 2016, the number of dengue cases reported in Rio went up by 320 per cent and 1150 per cent compared with 2015 and 2014 respectively.

“In the specific neighborhood of the Olympic Park (Barra da Tijuca) there have been more dengue cases in just the first quarter of 2016 than in all of 2015,” they write.

Transmission peak

But a study published on April 12, 2016 in the journal PLOS Neglected Tropical Diseases has for the first time documented 364 suspected cases and 119 laboratory confirmed cases of Zika in the State of Rio de Janeiro between January 1, 2015 and July 31, 2015.  The outbreak peaked during May-June 2015, before declining by July.

The months February to April is marked by warm and rainy season in Rio and provides an ideal situation for the breeding of mosquitoes.  So the peak of cases occurring in May-June is very much to be expected and is in line with what was witnessed in Yap Island (Micronesia) in 2007 and French Polynesia. Probably, we might see a similar trend this year as well in Brazil, and Rio in particular.


Aedes aegypti population will be the lowest during August. Photo: James Gathany/CDC

“There is a possibility that during the 2015 May-June outbreak in Rio de Janeiro a high percentage of the population also became infected with Zika, so a high percentage of the population became naturally immunized. The importance of this observation is that it will be more difficult to sustain Zika transmission among the population of Rio de Janeiro during the foreseeable future,” Dr. Peter Hotez Dean of the National School of Tropical Meidicine, Texas Children’s Hospital Endowed Chair in Tropical Pediatrics wrote in Baylor College of Medicine blog.

With August being the coolest month in Rio, the mosquito population will be at its lowest. “This finding may also explain why the 2015 epidemic in Rio died heading into July and August,” Dr. Hotez writes. But that does not mean that the risk of contracting Zika virus infection is nil during the Olympics.


Since the possibility of microcephaly is present when pregnant women are infected by Zika virus, it is best for them to follow the WHO advice and not travel to countries where Zika virus transmission is ongoing, including Rio.

“WHO advises pregnant women not to travel to areas with ongoing Zika virus transmission. This includes Rio de Janeiro. Pregnant women’s sex partners returning from areas with circulating virus should be counselled to practise safer sex or abstain throughout the pregnancy,” the WHO said in a release.

While the WHO asks couples returning from areas where transmission of Zika virus is known to occur to practise safe sex or abstain from sex for at least 4 weeks* after their return, the CDC insists on 8 weeks of abstinence or safe sex. The difference in the guidance provided by the WHO and CDC on sexual transmission reflects how little we know of Zika virus and the probability of transmission.

But if men develop symptoms then they should adopt safer sex practices or consider abstinence for at least six months, says the WHO.

The Zika virus persists in blood for only about a week.  It persists in urine for about two weeks.  It persists in saliva for about a week.  “That’s been studied.  Semen is an unknown.  There have been reports of virus persisting, not necessarily live virus but at least parts of virus, for up to two months,” Dr. Frieden said.

Mosquito control

Conventional measures to control A. aegypti mosquito population will be very difficult. Besides the eggs ability to survive even under harsh conditions for up to a year, controlling Aedes aegypti is particularly difficult. It is the cockroach of mosquitoes.

“I vividly remember in a trip to Puerto Rico, our lab team had set up a laboratory that hatched [A. aegypti] mosquitoes.  And they were testing them for resistance.  We put them in a bottle coated with insecticide to see whether they’re knocked down or not.  And to see them in a bottle that had been coated with what should be a very effective insecticide happily flying around minute after minute, hour after hour, shows us how important it is that we improve the methods we have of controlling mosquitoes,” Dr. Frieden told journalists.

Diagnosing Zika is hard but CDC has made some remarkable progress. CDC has optimised testing so we now have a “rapid, highly sensitive test that can be used in urine or blood that can pretty accurately detect the virus in someone who is acutely infected.  We’ve disseminated them to a hundred labs around the U.S. and in countries around the world,” Dr. Frieden said.  CDC has also developed “cutting edge technologies, virus-like particles and chimeric solutions that can knock down the spread of diseases spread like Zika by half, very simply, at a low cost.”


In its May 30 Interim guidance report, the WHO changed the duration during which couples should adopt safer sex practices or consider abstinence from 4 weeks to 8 weeks upon return. According to the report, though the “exact incubation period for Zika virus is still unknown, this period [of 8 weeks] is based on the estimated upper limit of the incubation period of 14 days for related flaviviruses. To this period, three times the longest published period of viremia after symptom onset (11 days) has been added, and additional time allowed for variability in individuals’ immune systems. This was also the approach adopted by the United States Centers for Disease Control and Prevention”.