YF virus is transmitted to humans through the bite of an infected mosquito, primarily Aedes or Haemogogus species. The incubation period is 3 to 6 days. Humans infected with YF virus experience the highest levels of viremia and are infectious to mosquitoes shortly before the onset of fever and for 3 to 5 days afterwards. Because of the high level of viremia in humans, bloodborne transmission of YF virus can occur through transfusion of blood products, intravenous drug use and needlestick injuries. Probable transmission of vaccine strain YF virus from a mother to her infant through breastfeeding has been reported. Vaccine-associated viremia occurs 4 to 10 days after primary YF vaccination and lasts for up to 5 days. Sustained transmission is not possible in Canada because the recognized mosquito vectors are not present.
A traveller’s risk for acquiring YF is determined by multiple factors including: immunization status, use of personal protection measures against mosquito bites, location of travel, duration of exposure, activities while travelling, and local rate of virus transmission. The risk for acquiring YF is low for most travellers, particularly those staying in highly developed major urban areas. Greater risk exists for travellers who:
- visit rural or jungle areas;
- stay for longer periods of time; and
- participate in outdoor activities such as recreation or fieldwork.
For example, risk is greater for travellers who stay in a rural area of an endemic country for over two weeks, whereas risk is low for travellers staying in an urban area in a transitional country for one week.
Source : Public Health Agency of Canada