Study suggests a single dose of JYNNEOS vaccine may lessen the severity of mpox illness

In a recent Morbidity and Mortality Weekly Report (MMWR) posted on the United States of America Center for Disease Control and Prevention (USA-CDC) website, researchers identified mpox (previously called monkeypox) patients who contracted the disease at or after 14 days of receiving one dose of the JYNNEOS vaccine. They compared their demographic and clinical characteristics with unvaccinated people who contracted mpox. Additionally, they drew comparisons with the vaccine-eligible population of all 29 participating jurisdictions in the USA.

Study: Demographic and Clinical Characteristics of Mpox in Persons Who Had Previously Received 1 Dose of JYNNEOS Vaccine and in Unvaccinated Persons — 29 U.S. Jurisdictions, May 22–September 3, 2022. Image Credit: Inside Creative House/Shutterstock
Study: Demographic and Clinical Characteristics of Mpox in Persons Who Had Previously Received 1 Dose of JYNNEOS Vaccine and in Unvaccinated Persons — 29 U.S. Jurisdictions, May 22–September 3, 2022. Image Credit: Inside Creative House/Shutterstock


Mpox cases continually rose worldwide in 2022, including in the USA. There were 29,133 mpox cases by November 14, 2022, with 95% of cases occurring among males. Earlier in 2019, the USA Food and Drug Administration (FDA) approved two 0.5 mL doses of the JYNNEOS vaccine for subcutaneous administration to prevent mpox and smallpox. Later, in August 2022, the FDA authorized the JYNNEOS use, a modified vaccinia Ankara (MVA) vaccine, intradermally at a dosage of 0.1 mL per dose.

About the study

In the present study, researchers pursued evidence that the JYNNEOS vaccine offered some protection against the mpox disease, especially they confirmed the duration for which the one dose of this vaccine conferred protection. The study covered people with probable and confirmed mpox disease from 29 jurisdictions in the USA who contracted it between May 22 and September 3, 2022.

The team considered those who had received a single dose of the JYNNEOS vaccine ≥14 days before contracting mpox disease as vaccinated; and those who had not received the vaccine during the 2022 outbreak or who fell ill before their first vaccination as unvaccinated. The team collected data from the USA immunization registries to analyze the demographic characteristics of the vaccinated vs. unvaccinated persons with mpox. They used Pearson’s chi-square or Fisher’s exact or Wilcoxon rank-sum tests, as applicable, to draw the demographics-related comparisons.

To compare the clinical characteristics of the two study groups, they relied on self-reported clinical symptoms data collected using a standardized data collection form. Based on the clinical symptoms, the team computed odds ratios (OR) and 95% confidence intervals (CI) which they used to compare the clinical characteristics of the two cohorts of mpox patients.

Notably, the immunization registries in the USA collect demographic & clinical characteristics of all the registered individuals, including their vaccination and medical history, plus exposures, and transmit the linked data to CDC. Finally, the team conducted sensitivity analyses covering mpox-infected cases who received a single JYNNEOS dose ≥22 days before disease onset.

Study findings

During the study period between May 22 and September 3, 2022, 14,504 mpox cases occurred across 29 jurisdictions in the USA. However, the researchers included only 6,605 (45.5%) mpox cases in the study analysis. Post-vaccination, the average time from vaccination to disease onset was 23 days. Specifically, 6,329 and 276 individuals, i.e., 95.8% and 4.2% of the study population, were unvaccinated and contracted mpox disease ≥14 days after receiving one dose of JYNNEOS, respectively.

The researchers observed a significant age distribution variation between vaccinated mpox patients and the vaccine-eligible population but not between them and unvaccinated mpox patients. Accordingly, over 91% of vaccinated mpox cases fell into the 18 to 49-year age group. Regarding racial origin, 68.2% and 49.9% of vaccinated and unvaccinated mpox patients self-reported their White race, respectively.

Among 345,220 individuals eligible for the single dose of the JYNNEOS vaccine, only 46.7% identified as White. On the contrary, 12.4% of the vaccinated and 30.9%  of unvaccinated patients self-reported that they belonged to the Black race, respectively. Mpox disease had disproportionately affected the male population in the USA, with 259 and 5,710 cases in vaccinated and unvaccinated people, respectively.

The researchers had data on a minimum of one clinical symptom for 202/276 and 5,326/6,329 vaccinated and unvaccinated mpox cases, respectively. While vaccinated people most commonly reported rash, pruritis, and enlarged lymph nodes, unvaccinated people reported having rash, fever, and malaise.

The odds of rectal symptoms, including proctitis, tenesmus, bleeding, and pain, were similar, but that of systemic symptoms, such as fever, malaise, chills, etc., were markedly lower among vaccinated vs. unvaccinated mpox patients.

In 55% of vaccinated and 46.7% of unvaccinated patients, the genital and perianal area was the most common rash location. Vaccinated individuals, on average, had two, while unvaccinated individuals had three rash locations. While no deaths occurred among any participating individual infected with mpox, the researchers noted that more unvaccinated than vaccinated (8% vs. 2%) individuals sought hospitalization. The results of the sensitivity analyses were similar to that of primary analysis findings, irrespective of days before illness onset (≥22 days vs. ≥14 days).


To summarize, 276 mpox-infected cases in this study who had received one dose of the JYNNEOS vaccine greater than or 14 days before disease onset had similar but less frequent clinical symptoms than 6329 unvaccinated mpox patients. Although these clinical signs and symptoms were comparable to those identified early in the outbreak, vaccination with JYNNEOS substantially weakened the severity of mpox infections and the odds of systemic disease symptoms.

Both vaccinated and unvaccinated mpox cases, however, suffered most commonly from rashes in genital and perianal locations suggesting that sexual transmission was perhaps the most common mechanism of mpox disease transmission. The vaccinated individuals had fewer rash locations, indicating that even partial vaccination prevented the spread of the virus from the inoculation site.

The observed predominance of people of White ethnic origin among vaccinated mpox patients reflected the ongoing racial disparities in access to the JYNNEOS vaccine and healthcare facilities in general in the USA. This finding also indicates differential acceptance of the vaccine among different racial communities. Overall, these disparities could have decreased the severity of mpox disease in vaccinated White persons.

The CDC has been monitoring the efficacy and safety profile of mpox vaccines, such as JYNNEOS. The data so collected is used by the CDC to guide and educate public health officials about non-vaccine-related prevention strategies. However, more importantly, the CDC surveillance could help ensure that people remain motivated to complete the two-dose JYNNEOS vaccination regimen, which, in turn, could prevent a mpox outbreak. 

Although the cases of mpox ≥14 days after JYNNEOS vaccination are fewer, the duration of protection conferred by its one vaccine dose is unknown. The study results support the benefit of vaccination with one dose of the JYNNEOS vaccine in attenuating mpox disease.

There is also an urgent need to encourage more and more people to complete the two-dose JYNNEOS vaccination series till the majority at risk of mpox attain optimal immune protection from the second dose. In addition, people could also adopt other prevention strategies recommended by the CDC.

Journal reference:
Neha Mathur

Written by

Neha Mathur

Neha is a digital marketing professional based in Gurugram, India. She has a Master’s degree from the University of Rajasthan with a specialization in Biotechnology in 2008. She has experience in pre-clinical research as part of her research project in The Department of Toxicology at the prestigious Central Drug Research Institute (CDRI), Lucknow, India. She also holds a certification in C++ programming.


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