From Your Local Epidemiologist:
The WHO recommends vitamin A for people who contract measles because:
- the WHO serves 197 countries, many of which are low- and middle-income nations where Vitamin A deficiency is a major public health issue (particularly among children);
- Vitamin A plays a crucial role in immune dysfunction (T cell response, antibody production); if someone is deficient in vitamin A, their immune system is not functioning at its best capacity.
Studies have shown that:
- Vitamin A supplementation reduces measles-based mortality by 12% (over 1 million children studied across 19 countries) (source)
- There was no overall mortality reduction found from vitamin A in an analysis of eight RCTs (source)
- In three studies focused on African children under two years old, vitamin A supplementation reduced mortality by 79% (source)
However, only 0.3% of the US population has a vitamin A deficiency. Similar studies from other high-income countries with low levels of vitamin A deficiency suggest that the vitamin doesn’t make that much difference on measles. There is some data to suggest that vitamin A levels may drop during a measles infection, and when administered in cotrolled doses under the supervision of a doctor, it is a low risk intervention. But it’s unclear how much it actually helps.
TL;DR: You can overdose vitamin A (it leads to liver damage and other serious health issues), and it does not prevent measles.