Researchers have found evidence for “substantial” monkeypox transmission before symptoms appear. A study published in the British Medical Journal found that pre-symptomatic transmission was detected up to a maximum of four days before the onset of symptoms. The researchers estimate that more than half (53%) of transmission incidents occurred in this pre-symptomatic phase. This is the first study to provide evidence that transmission can occur prior to the appearance of physical symptoms.
Some common monkeypox symptoms that precede rash development include fever, malaise, and muscle aches. There are roughly 70,000 cases globally.
Researchers at the UK Health Security Agency conducted routine surveillance and contact tracing data for 2,746 individuals who tested positive for monkeypox virus in the UK between May and August 2022. The average patient age was 38 years and 95% reported their sexual orientation as gay, bisexual, or men who have sex with men, but all populations regardless of age or sexual orientation are at risk of monkeypox.
The two key measures were serial interval and incubation period. The researchers analyzed the primary case patient’s time from symptom onset to noticeable symptoms experienced by the secondary contact) and incubation period (time from exposure to symptom onset). The researchers conducted statistical analysis on exposure and symptom onset dates from individuals to their contacts collected via contact tracing case questionnaires and adjusted models for several biases common to virus outbreaks (changes in infection rates over time).
The mean incubation period in one model was estimated to be 7.6 days and 7.8 days in the second model, while the estimated mean serial interval was 8 days in one model and 9.5 days in the second. Both model median serial intervals were between 0.3 and 1.7 days shorter than the median incubation period. This evidence suggests considerable transmission is occurring before the appearance or detection of symptoms.
An additional research step involved analysis of individual level patient data also confirmed that transmission happens before symptom detection. This analysis showed 10 out of 13 case-contact patient pairs indicated pre-symptomatic transmission and the maximum time between transmission detection and symptom onset was 4 days. The researchers believe a 16-23 day isolation period would be required to detect 95% of potentially infected people.
The observational findings have several limitations, such as relying on contact tracing to identify the correct case-contact pairs and the self-reported data on date of symptom onset. Results may not be generalizable to other populations with different transmission patterns. The findings have important implications for isolation and contact tracing policies. The researchers emphasize that backward contact tracing strategies should account for a pre-symptomatic infectious period when identifying the contacts of confirmed cases.
This study provides insights into the transmission dynamics that will help inform policy decisions and future interventions. The study highlights that vaccination may be more cost effective than managing expenses related to hospital care, sick leave, and other consequences of viral infection.