Monkeypox: Presymptomatic Transmission May Be Substantial

The incidence of monkeypox virus infection in the United Kingdom was found to be highest on July 9, 2022. Analysis of disease transmission dynamics showed short serial intervals occurred more frequently than short incubation periods, indicating substantial presymptomatic transmission. These study findings were published in BMJ.

Researchers conducted a contact-tracing study to evaluate the dynamics of monkeypox virus transmission in the UK between May and August 2022. The analysis included 2746 participants with polymerase chain reaction-confirmed monkeypox infection. The primary outcomes were the incubation period and serial interval of monkeypox infection, determined via questionnaires completed by the participants and analyzed with 2 bayesian time-delay models. The first model corrected for interval censoring and the second model corrected for interval censoring, right truncation, and epidemic phase bias. Generalized additive models were used to estimate growth rates of infection by date first reported to the UK Health Security Agency.

Among participants included in the analysis, the mean (SD) age was 37.8 (9.1) years, and 95% identified as gay, bisexual, or other men who have sex with men. In addition, 31% of participants reported at least 10 or more sexual partners within the previous 3 months, and more than half were diagnosed with a sexually transmitted infection within the past year.

Analysis of the first and second bayesian time-delay models indicated that the estimated mean incubation period of monkeypox virus was 7.6 (95% credible interval [CI], 6.5-9.9) and 7.8 (95% CI, 6.6-9.2) days, respectively. Of note, data in both models were best fit using Weibull distribution.

Further analysis of both the first and second models showed that the estimated mean serial interval time (8.0 and 9.5 days, respectively) was increased compared with the estimated mean incubation period. Despite this finding, short serial intervals were found to be more common than short incubation periods.

Of 13 participants with linked contact-tracing data, 10 had documented presymptomatic monkeypox transmission. The maximum time between transmission detection and symptom onset was 9 days, which decreased to 4 days when the analysis was restricted to only participants with personal identifiable data (n=4).

Evaluation of the instantaneous growth rate also was performed, with results suggesting that the monkeypox epidemic reached its peak in the UK on July 9, 2022. A post-exposure isolation period ranging between 16 and 23 days was found to be adequate for detecting 95% of incident infections.

Limitations include the use of patient-reported data, the reliance on contact tracing to identify case-contact pairs, and potentially limited generalizability for assessing outbreaks of monkeypox virus in other settings.

“The global transmission of the monkeypox virus has been on a scale not previously seen outside of Central Africa,” the researchers noted. “Although case numbers are declining,” they added, “increased international transmission would facilitate infection importation and might drive stochastic outbreaks even if vaccination in local networks limits transmission.”