One dose of Mpox vaccine series appears to provide partial protection

According to the Canadian observational emulate target trial, a single dose of modified vaccinia Ankara-Bavarian Nordic (MVA-BN; Jynneos) mpox vaccine was moderately effective in preventing mpox infection.

Sharmistha Mishra, MD, MSc, of the University of Toronto and colleagues reported that among 3,204 men who received the first dose of MVA-BN vaccine during the 153-day study period, 21 were diagnosed with laboratory-confirmed MPOX compared with 50 of 3,204 unvaccinated men (HR 0.42, 95% CI 0.25-0.69), resulting in an estimated single-dose vaccine efficacy of 58%. BMJ.

This corresponds to 0.09 infections per 1,000 person-days (95% CI 0.05-0.13) in the vaccinated group and 0.2 infections per 1,000 person-days (95% CI 0.15-0.27) in the unvaccinated group during the study period.

“These results were not surprising, as previous observational studies have reported efficacy of a single dose of MVA-BN against MPOX infections ranging from 36% to 86%,” said study co-investigator Jeff Kwon, MD, PhD, of the University of Toronto. Medpage Today.

“This study reinforces the benefits of MPOX vaccination with MVA-BN and reaffirms that a single dose of the vaccine provides at least moderate protection to MPOX-vulnerable communities,” said Dr. Anirudh Hazra, an infectious disease specialist at Howard Brown Health in Chicago. Medpage Today.

Hazra noted that the study also aimed to reduce bias and confounding factors seen in previous observational studies. But “the final results are not that surprising,” he said. “The calculated vaccine efficacy is broadly in line with what we would currently expect to see with a single dose of MVA-BN.”

In Canada and the United States, the MVA-BN vaccine is Recommendation It is administered as a two-dose vaccine, given 28 days apart. However, during the first MPOX outbreak in 2022, the vaccine will be administered as a single dose or Vaccine supply is declining Due to concerns about limited vaccine supplies.

In August, the World Health Organization Second Global MPOX Emergency Declared In response to the rapid increase in MPOX lineage I variants in Africa and the increasing number of pediatric cases.

Mishra and his colleagues wrote that Mpox infections in Canada and around the world will rise again in 2024, with most diagnoses occurring among people who have not yet been vaccinated or who have received only one dose of the vaccine.

“Given the moderate efficacy of a single dose, achieving high coverage with a full course of vaccine may be crucial to prevent and control ongoing infections around the world and to prevent major resurgences,” they commented.

“It is important that we now advise people who are at risk of MPOX infection, such as gay and bisexual people, men who have sex with men, and sex workers, to receive MVA-BN as soon as possible to have some protection against MPOX infection,” Kwon added, especially during the current global public health emergency.

Another MPOX vaccine in the United States, ACAM2000is a recently approved, single-dose vaccine.

The study authors noted that randomized clinical trials of the MPOX vaccine have not yet been conducted.

Mishra and two colleagues explained: Accompanying Articles Because clinical trials have been impractical and sometimes unethical in the face of a global health crisis, researchers have relied on observational studies to assess how well the MPOX vaccine works.

This was also an observational study, but the researchers used a targeted trial emulation design to incorporate features of a randomized trial, and they also performed sensitivity analyses to determine residual confounding.

The researchers noted that a vaccine's ability to prevent laboratory-confirmed infection is only one indicator of a vaccine's effectiveness: For example, the study did not assess the extent to which the MVA-BN vaccine reduced symptomatic infection or the severity of infection, or whether it induced herd immunity.

Still, “our findings suggest that the local MVA-BN vaccination program may have been a factor in slowing the spread of MPOX in the region in 2022,” the researchers said.

The study will be conducted during Canada's first MPOX surge from May 2022 to October 2022 and will match data from vaccinated men with unvaccinated controls on age, geographic region, previous HIV diagnosis, bacterial sexually transmitted infection (STI) diagnosis in the past 3 years, and vaccination other than MVA-BN in the previous year. The mean age of matched participants was 35 years, 66% lived in Toronto, and approximately 22% had previously been diagnosed with HIV.

To confirm the specificity of the association between vaccination and reduced infection rates, the researchers conducted a sensitivity analysis and found that vaccination was not associated with reduced MPOX infection rates during the first 14 days after vaccination (before the expected antibody response) or with diagnoses of bacterial sexually transmitted diseases that are not affected by the vaccine.

Because only a small number of study participants (13.7%) received the second dose by the end of the study period, the researchers were unable to assess the effectiveness of the two-dose series.

The authors acknowledge several limitations of the study. The final cohort consisted of less than 10% of first doses of MVA-BN vaccine and MPOX diagnoses in Ontario during the study period, resulting in a reduced sample size. Some patient information, such as previous smallpox vaccination and sexual exposure, was not available. Also, the study population may not have included at-risk men with inadequate access to health care, introducing selection bias. Finally, the study included only laboratory-confirmed infections, which may have resulted in additional cases being missed and an overestimate of vaccine efficacy.