Prior to the introduction of the measles vaccine (1963), there was essentially no reduction in the incidence of measles. Nowhere had any luck containing spread until the advent of vaccines.

The good note is that death rates declined due to globalization. People often get this mixed up though, and they think that the death rate fell due to nutrition and hygiene, but those factors make no sense to reference. Just think about how the death rate fell.

When measles was introduced to places like Iceland or the Faroe Islands or the Scottish Highlands or isolated Greek islands or rural communities in the American South, it was highly lethal. Then it became less lethal with exposure, early exposure survival, etc.

This happens so rapidly that it cannot be explained by improving nutrition or hygiene, and it is virtually all of the case fatality decline in these groups/areas/etc. Lack of vitamin A is important, meaningfully low vitamin A is quite rare, even in the developing world.

The hygiene argument makes particularly little sense because the only way it could work out would be through reducing coinfection Hygiene improvements did not reduce transmission, since measles is transmitted through the air Hand-washing, sewage treatment, clean water unrelated

Beyond the protective effects of globalization, being able to provide vitamin A when needed, being able to treat pneumonia and bacterial superinfections, etc. did help with the case fatality rate. But that's hardly what the hygiene hypothesis supposes happened.