The World Health Organization declared the world free of Smallpox on May 8, 1980. It is often touted as one of the foremost achievements of public health. Because of this eradication, the United States stopped its routine vaccination program for Smallpox in 1972.
Monkeypox, which is currently causing an outbreak in multiple counties, is another orthopox virus and is closely related to the variola virus that causes Smallpox. Because of the similarities between the viruses, the Smallpox vaccine has been shown to be 85% effective at preventing Monkeypox in clinical trials.
Since we stopped vaccinating for smallpox decades ago, it is not completely surprising to epidemiologists that we have a current monkeypox outbreak because we have had an increasingly susceptible population ever since.
Because of the previous vaccination program, many people 50 and older in the US have likely been vaccinated for Smallpox. A common question that people are asking is whether they are protected from Monkeypox if they were given the Smallpox vaccine in childhood.
Unfortunately, we do know that protection from the smallpox vaccine wanes over time. Smallpox vaccination can protect you at about 85% from monkeypox infection for about 3 to 5 years. After that time, its ability to protect you against infection decreases, but experts believe it would still protect against severe illness. However, a caveat is that at the time when the routine smallpox vaccine was being given, testing was not as advanced or precise as it is today, so there are things we still need to learn from the current outbreak about the vaccine effectiveness and its duration of protection.