Sorry, your 50-year old smallpox vaccine won't help you with monkeypox

Monkeypox is the next COVID. Well, not really. The two are hardly comparable. Monkeypox is related to smallpox (both are orthopoxviruses) although usually less severe and less contagious. Symptoms include fever, muscle aches, skin rashes, swollen lymph nodes and headache. It typically lasts from 2 - 4 weeks, and the fatality ratio is relatively low, in the region of 3 - 6%. It mainly occurs in west and central Africa, but there has been a rash [sic] of cases in the West, which is freaking people out. Many of the early cases occurred particularly, although not exclusively, among men who have sex with other men, but note: it is not a "gay disease", and it is not even sexually transmitted, but is spread by close contact, clothed or not. Over 200 confirmed or supected cases have been identified in over 20 countries, with more new cases being reported every day.

But wait, you say, I've heard that older people who have been vaccinated against smallpox as a child - as I think I have, although I have no proof - are protected against monkeypox. Well, sorry, but that is almost certainly not the case. 

Like with most vaccinations, immunity wears off over time. Routine smallpox vaccinations ended back in the early 1970s, some 50 years ago, even though smallpox worldwide was not eradicated until 1980. However, neutralizing antibodies decline significantly over 5 to 10 years, which is why the smallpox vaccine used to require boosters every 5 to 10 years. Because the disease was considered eradicated, though, obviously no-one bothered with boosters after the 1970s. 

Ergo, any smallpox vaccines you might have received already are not going to be any use at all to you now, 50 years later, either against smallpox or against monkeypox. Some older studies have shown that the smallpox vaccine was of some use in reducing the severity of the disease for up to 20 years, but even after 20 years its efficacy was minimal (and zero against actually catching the disease). So, after 40 or 50 years ... well, you catch my drift.

However, many countries do still have stocks of the smallpox vaccine, for emergency purposes, and so medical staff who may be exposed to the current monkeypox outbreak could use it to give some protection, as is currently happening in the UK. The monkeypox outbreak is relatively small, though, and because it is not that contagious (compared to COVID for example), it is expected to be controllable, so mass vaccinations are unlikely.