Obviously the risk of paralytic polio from OPV is very very low, but we kept giving it instead of switching to IPV for a chunk of time where the risk of paralytic polio from OPV was much higher than the risk of paralytic polio from polio.
Sad story for those affected, but it is more of a case of us using the OPV for too long rather than it being a bad idea in the first place.
By what metric was this change "too long?" I think there's multiple issues that played into the timing of the decision for conversion of OPV to IPV that makes it a bit more complex than a simple refusal to switch from one vaccine version to another.
1) The Cutter Incident with IPV. If you haven't heard about it, Paul Offit has a really
good book on it. Yes, IPV is still very much inactivated the same way now as it was in the 1950's, which I've always thought to be remarkable (in both good and bad ways). All it takes is a systematic failure of QC and a similar incident is theoretically possible (but I hope the secondary safety nets in place would prevent it from ever happening again).
2) Because of the success of OPV, only
one manufacturer produced the original IPV formulation in the US into the 1980's.
3) Of the vaccine associated paralysis when published in 1994, ~
15% did not result in significant or severe residual paralysis. On the other hand, not to ignore, 5 cases between 1980 and 1991 resulted in death.
4) There were serious concerns about the immunogenicity and production of the IPV even after the Cutter incident. It wasn't until the 1970's and 1980's that the modern version of IPV was formulated, and it wasn't approved by the
US FDA until 1987.
5) Nonetheless, it took further time to understand whether the modern IPV formulation would have sufficient long-lasting immunity. Until that was known, it was unclear if additional booster doses would be needed, whereas OPV was known to offer nearly life-long immunity.
6) Even into the 1990's, it was recognized that the newer IPV vaccine induced a
weaker mucosal immune response compared to OPV. This was a concern that could not be discarded given the potential for importation of the virus.
7) The last outbreak of paralytic polio occurred in 1979 in the US, but that doesn't rule out the potential for viral circulation in certain populations beyond that date (and just didn't cause paralytic polio).
8) In fact, as late as 1993, polio was known to
circulate in certain unvaccinated populations in Canada, but didn't cause poliomyelitis.
9) The last known case of non-OPV associated poliomyelitis to occur in the
Western Hemisphere was 1991 in Peru and Colombia. It wasn't until
2002 that Europe was deemed polio-free.
10) The ACIP and CDC decided to switch from OPV to a mixed series of
IPV + OPV in 1997, and the full conversion to IPV occurred
in 2000
One last fact I've always found incredible. One mutation is the difference between the attenuated polio strain in OPV versus a neuroinvasive strain. This is why OPV caused paralytic polio in rare cases (1 case per 2.4 million doses), all was needed was a single mutation.
www.cdc.gov
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