Average Monkeypox Incubation Period is 8.5 Days

Since the beginning of May 2022, monkeypox outbreaks have been reported in nonendemic areas, including the US. The monkeypox virus is believed to only be transmissible when an infected person is symptomatic, but what is the timeline from infection to presentation?

Using confirmed cases in the Netherlands, one study estimated the incubation period of monkeypox. The study, published in Eurosurveillance, emphasized the need to determine the incubation period of the recent outbreaks to conduct timely and effective contact tracing.

In the Netherlands, monkeypox was classified as a group A notifiable disease on May 21, 2022. The investigators conducted data surveillance from this date until May 31, 2022, at which point a total of 31 monkeypox cases were laboratory-confirmed. All infections occurred among men who have sex with men (MSM), and they ranged in age from 23-64 years.

Out of the 31 cases, 18 had reported the symptom onset date and the suspected date(s) of their viral exposure. The investigators fitted parametric distributions to the observed incubation periods of the 18 cases with documented exposure and symptom onset histories. They utilized a likelihood-based approach, allowing for exposure to be a single point in time or a time interval. The investigators compared 3 different parametric distributions, the lognormal, the Gamma, and the Weibull distribution, before selecting the distribution that best fit.

The average incubation period was found to be 8.5 days, with the 5th percentile at 4.2 days, the 95th percentile at 17.3 days, and the 97.5th percentile at 19.9 days to symptom onset. Thus, the investigators concluded that the currently recommended 21-day period of monitoring, isolating, and quarantining close contacts of confirmed monkeypox cases was very reasonable.

The investigators estimated the proportion of monkeypox cases that would become symptomatic after the 21-day period to be 2%. However, as outbreaks continue and transmission routes are still under investigation, it is essential to continue tracking the time from monkeypox infection to symptom presentation.

Notably, the monkeypox incubation period has been reported to differ by mode of transmission. For non-invasive exposure, such as intact skin-to-skin contact or droplet transmission, the typical incubation duration is 13 days. For complex or invasive exposures, such as contact with broken skin or mucous membranes, the average incubation period shortens to 9 days. These numbers are consistent with the incubation period of smallpox.