The number of cases of measles in total might have been severely underreported, but deaths and hospitalizations? It's one thing if the parents keep home a kid sick with measles but not seriously ill and don't bother to file an official report. But if the kid dies, surely the coroner is most likely going to find out what killed him, and if he's taken to the hospital, the hospital staff will diagnose him and record it in the hospital records.
This was before ICD coding, among other things. If the proximate cause of death was pneumonia, then there was an excellent chance that the attending doc would have written it off to "pneumonia" rather than "measles" or "measles pneumonia." Things changed when measles became rarer and the reporting requirement had more teeth (IIRC, my parents knew that there was nothing the family doc could do for measles so when I got it they never even took me to him.)
Then there's the less-common sequelae such as subacute sclerosing panencephalitis. Likely not reported at all, thanks to the time delay. Similarly for the more common forms of encephalitis -- death by encephalitis was much more common for a lot of reasons and measles was just one of them.