Last September, in a blog called CDC EID Journal: Monkeypox in Patient Immunized with ACAM2000 Smallpox Vaccine During 2022 Outbreak, I opened with the following paragraph.
Although it has been often stated by public health officials (and the vaccine manufacturers), it bears repeating; we really don't know how effective the JYNNEOS(TM) vaccine approved in 2019 (and the older ACAM2000 smallpox vaccine) will be against the strain of Monkeypox currently spreading globally.
This from the CDC website at that time:
Because Monkeypox virus is closely related to the virus that causes smallpox, the smallpox vaccine can protect people from getting monkeypox. Past data from Africa suggests that the smallpox vaccine is at least 85% effective in preventing monkeypox. The effectiveness of JYNNEOS(TM) against monkeypox was concluded from a clinical study on the immunogenicity of JYNNEOS and efficacy data from animal studies.
Smallpox and monkeypox vaccines are effective at protecting people against monkeypox when given before exposure to monkeypox. Experts also believe that vaccination after a monkeypox exposure may help prevent the disease or make it less severe.
Today, via a CDC HAN Health Update, we're learning that the vaccine may not be as effective as originally hoped. This HAN alert from Chicagohan.org.
RESURGENCE OF MPOX - Provider Update: May 9, 2023
Publish Date:05/09/2023 06:33:11 pm
Level of Alert: HIGH
Today the CDC has issued a HAN Health Update for clinicians, advising them that while vaccination is still strongly recommended (it likely still prevents some cases, and may reduce the severity of breakthrough infection), they should expect to see new cases even among previously vaccinated individuals.
Small genetic changes could favor adaptation to a human host, and this potential is greatest for pathogens with moderate transmission rates (such as MPXV) (40). The ability to spread rapidly and efficiently from human to human could enhance spread by travelers to new regions.In a 2020 report, published by the Bulletin of the World Health Organization, researchers warned that our waning immunity to smallpox put society at greater risks of seeing Monkeypox epidemics (see WHO: Modelling Human-to-Human Transmission of Monkeypox).
While Mpox is currently disproportionately affecting gay, bisexual, and other men who have sex with men (MSM), there are no guarantees that pattern won't change over time.