The respiratory syncytial virus (RSV) vaccine tested in the 1960s was a killed virus vaccine that led to worse outcomes upon natural virus exposure compared to non-vaccine recipients.

“this is a phenomena that was found originally with the respiratory syncytial virus vaccine the RSV vaccine that was a killed virus vaccine a formal and inactivated vaccine that was tested by the National Institutes of Health at Children's Hospital in Washington in the 1960s where vaccine recipients actually did worse than the non vaccine recipients after they were exposed to the natural virus living in the community and the mechanism is not still entirely clear but those who were vaccinated had more hospitalizations than they were even two deaths”