A member of the pneumovirus genus, respiratory syncytial virus (RSV) is a highly important cause of lower respiratory tract disease in infants. It replicates at that site in several species, but the mouse and cotton rat are the main models, though neither accurately demonstrate the symptoms of the human disease. Shortly after its discovery, virus grown in African green monkey kidney tissue culture was inactivated to form a vaccine. The rationale for this approach was the prior experience with both the influenza and poliovirus vaccines which had been prepared using similar approaches, though an early batch of the latter had been found to be incompletely inactivated. However, the candidate RSV vaccine not only failed to confer protection against a subsequent RSV infection, but the disease which RSV vaccinees experienced on subsequent infection with the wild-type virus was unexpectedly severe, many vaccinees developing pneumonia or bronchiolitis with some deaths. Both humoral and cell-mediated reactions may have contributed to this result. Many attempts to make an effective subunit vaccine have since been made, but currently, a live, attenuated vaccine may turn out to be the best preparation.
To avoid a possible outbreak in 1976 of swine influenza (the viral subtype which caused the 1918 influenza pandemic in which an estimated 20 million people died), a rush program was instituted in the USA to vaccinate many millions of US citizens with a vaccine containing this serotype. The vaccine went through the usual tests during which it was administered to many volunteers. But in a total of 41.5 million people who finally received the vaccine, about 500 cases of Guillain-Barre syndrome occurred in vaccinees within a few weeks of vaccination, an incidence of about 1/80 000 recipients. The chances of detecting such a relationship in clinical trials is negligible. This relationship had not been seen previously or since, and it has been speculated that such an effect might only be seen in an extremely large, rushed program.
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