The letter also claims that “most of South Asia and Africa” are “as yet unaffected places”. That is not true either. Many studies, going back to the 1940s, have shown that humans, and often also monkeys, in several African and Asian countries have antibodies to the Zika virus, and we have direct evidence of Zika circulating in both West Africa and South East Asia within the last ten years. These studies need to be repeated to determine the current levels, but it is very likely that many parts of the tropical world have already seen Zika virus in the past and there will probably be considerable herd immunity to the local Zika variant in the human population.
Will that previous exposure be protective against more exotic strains of Zika? The likely answer is “yes”. The Brazilian and African varieties of Zika are about 12-13% different in terms of the genetic variants accumulated since their last common ancestor. The corresponding figure for yellow fever virus, another member of the flavivirus family that includes Zika, is just under 22%. Yet a single yellow fever vaccination will protect against all variants of yellow fever and for a long period of time, so it would be very unusual if exposure to African or South-East Asian Zika did not similarly generate immunity against Brazilian Zika.
Brazil has many of the typical diseases found in the tropical world, including malaria and dengue as well as levels of HIV about three times higher than the UK. These are things that travellers and athletes need to prepare for. All are clinically more serious than Zika, but none has been proposed as a reason to cancel the event.