Although the enzyme might indicate the presence of a virus, such as a retrovirus that could cause infection and possible illness in humans, health authorities say no illness has been detected. Current evidence suggests the source of the enzyme is the chicken cells used to produce the vaccine, not a virus. In a medical advisory sent to epidemiologists, Health Canada noted that no retroviruses from chickens or other species of birds are known to be able to cause infection in humans or human cells. Many years ago truly infectious avian retroviruses were detected in the cell line used to produce yellow fever vaccine, but a 1970s' study of people who received that vaccine during WW II revealed no evidence of a health risk.
Worldwide vaccination recommendations remain unchanged, although Health Canada and the World Health Organization are among the international health authorities that continue to monitor the situation. The routinely monitored vaccines have been used worldwide for many years without evidence of serious adverse events, the Health Canada advisory said, and have greatly reduced the burden of the diseases they prevent. It added that parents concerned about the measles-mumps-rubella vaccine given to children should be assured that it has not changed and remains both safe and effective.