One, named ACAM2000, is a modern version of the age-old smallpox vaccine, made from unmodified vaccinia virus, that helped eradicate that disease from humans, a feat WHO celebrated as complete in 1980. FDA approved ACAM2000 in 2007, and the United States has enough of it in its Strategic National Stockpile (SNS) to vaccinate the country’s entire population if there is a bioterror attack with smallpox. Some of the vaccine has been used “off label” for monkeypox in the current outbreak. But vaccinia copies itself after vaccination, which can lead to serious disease, especially in people who have compromised immune systems from HIV—which has a high prevalence among MSM—or other factors.
MVA, the virus in Bavarian Nordic’s vaccine, does not replicate in the body and is much safer, which makes it the preferred vaccine for the monkeypox outbreak. Known as Jynneos in the United States, Imvanex in Europe, and Imvamune in Canada, MVA is the only vaccine FDA has explicitly approved for monkeypox. Because monkeypox is so rare, the company had no human efficacy data when it applied for approval from FDA; the agency granted its license in 2019 in part based on studies showing the vaccine protected animals from monkeypox...
In a study published in The New England Journal of Medicine
in 2019, Chaplin and colleagues compared immune responses in people given either MVA or ACAM2000. With smallpox virus, levels of neutralizing antibodies are seen as a key indicator of protection, so the same idea extends to monkeypox virus. The researchers found that 14 days after a single dose, the levels of antibodies that neutralize the monkeypox peaked with MVA, at a level nearly identical to that triggered by ACAM2000. Given that the vaccines are presumed to work postexposure if given within 2 weeks of contact, this suggests that a single dose of MVA could prevent many cases of disease and slow spread.
In a 2008 monkey study led by Moss, published in the Proceedings of the National Academy of Sciences
, researchers compared preexposure vaccination against monkeypox from a single dose of MVA and one dose of a cruder version of ACAM2000 called Dryvax. Because MVA does not make copies of itself, the team gave it at a higher dose—similar to what’s used in the Bavarian Nordic shot today—than the Dryvax vaccine.
Both vaccines worked well, but MVA appeared to work faster. Levels of neutralizing antibodies and CD8 cells—critical immune actors that destroy infected cells—both rose more rapidly with MVA, which Moss and coworkers concluded was because it took time for Dryvax to replicate to levels that matched the initial shot of MVA. What’s more, when they injected the animals with the monkeypox virus just 4 days after vaccination, it grew less well in monkeys vaccinated with MVA.
In a statement to Science
, CDC stressed that a single dose of the vaccine hasn’t been studied in any outbreak and cautioned that it is “challenging” to extrapolate from immune responses in earlier studies and protection in animal studies.