Requiring patients diagnosed with monkeypox to self-isolate to control the spread of the virus might be less effective than previously thought, after research suggested "substantial" pre-symptomatic transmission.
A UK investigation, published in The BMJ, found evidence of monkeypox virus transmission taking place up to 4 days before the onset of symptoms and estimated that more than half of transmission occurred in the pre-symptomatic phase.
Previous research on pox viruses did not rule out transmission prior to symptoms, but the researchers from the UK Health Security Agency said their work represented the first evidence to support this.
The scientists focused on the serial interval of time from symptom onset in the primary case to symptom onset in the secondary case, and the incubation period. That information was extracted from routine surveillance and contact tracing data for 2746 people who tested positive for monkeypox virus in the UK between May 6 and August 1, 2022. The average age of the cohort was 38 years, and 95% had reported being gay, bisexual, or men who have sex with men.
Incubation Period and Serial Interval
The mean incubation period of monkeypox was 7.6 days (95% CI, 6.5 to 9.9) in one model used and 7.8 days (95% CI, 6.6 to 9.2) for another model. The estimated serial interval was 8 days (95% CI, 6.5 to 9.9). for one model and 9.5 days (95% CI, 7.4 to 12.3) in the other.
For both models, the median serial interval was between 0.3 and 1.7 days shorter than the median incubation period, which the researchers said indicated a significant space for monkeypox virus transmission before symptoms appeared. When they analysed individual patient-level data, results seemed to confirm the findings, with 10 out of 13 case-contact patient pairs reporting pre-symptomatic transmission.
They estimated that 53% (95% CI, 43% to 62%) of transmission occurred in the pre-symptomatic phase. They cautioned, however, that "since serial intervals depend on the incubation period, this finding is an approximation of the proportion of infections due to pre-symptomatic transmission".
Based on the results, the researchers suggested that an isolation period of 16 to 23 days after exposure to the monkeypox virus would be required to identify 95% of infected individuals.
The study authors acknowledged some weaknesses in their analysis, including its observational nature and the reliance on contact tracing to identify the correct case-contact pairs.
'Important Implications' for Infection Control
In a linked editorial, researchers based in the US, UK, and Nigeria said that if the findings were backed by further research, "then pre-symptomatic transmission, or transmission before symptoms are detected, would have important implications for infection control globally". In particular, "postexposure or 'ring' vaccination of contacts identified only through individuals with symptoms, could be inadequate", they argued.
Separately, Dr Boghuma Kabisen Titanji, assistant professor of medicine at Atlanta's Emory University in the US, said the scientists had carried out a "robust" analysis, using adjustments to reduce the risk of bias. She told the Science Media Centre: "The point the authors make about asymptomatic transmission based on the serial interval being shorter than the incubation period is interesting and convincing based on the analysis performed and also confirms observations made from clinical studies where monkeypox has been detected in samples collected for routine STI screening from asymptomatic individuals. This needs confirmation by more studies but has implications for vaccination-based disease elimination strategies which should be seriously considered."
Dr Titanji said questions requiring answers were "what proportion of cases are asymptomatic, and how much do these cases contribute to seeding new transmission chains?".
The study authors declared no financial relationships with any organisations that might have an interest in the submitted work in the previous three years. Dr Titanji declared no conflicts of interest.
Click here for all of the latest monkey coverage
Lead Image Credit: E+/Getty Images
