Monkeypox is transmitted before symptoms appear

A study published in The BMJ has seen evidence of “substantial” transmission of monkeypox before symptoms appear or are detected (known as presymptomatic transmission).

The transmission is detected up to a maximum of four days before from the onset of symptoms, and the researchers estimate that more than half (53%) of transmission occurred in this pre-symptomatic phase, which means that many infections cannot be prevented by asking people to isolate themselves at the time who notice the first symptoms.

In a linked editorial, the researchers say that if these findings are supported by other studies, presymptomatic transmission “would have important implications for infection control globally.”

Since the international outbreak of monkeypox in May 2022, more than 70,000 cases have been registered worldwide, with Spain being one of the most affected countries.

Although the number of cases is now declining, it is still important to understand the “transmission dynamics” of the virus – for example, how it spreads from person to person and how quickly symptoms appear – to help inform policy decisions. and future interventions.

While previous research on smallpox viruses has not ruled out transmission before symptoms, this work represents the first evidence that this is the case.

To explore this further, researchers from the UK Health Security Agency set out to analyze the transmission dynamics of the UK monkeypox outbreak.

Their findings are based on routine surveillance and contact tracing data for 2,746 people who tested positive for the monkeypox virus in the UK between May 6 and August 1, 2022.

The two main measures of interest to the investigators were serial interval (time from symptom onset in the primary case patient to symptom onset in the secondary contact) and incubation period (time from exposure to onset of symptoms).

To estimate it, they linked information about exposure and symptom onset dates for these people to their contacts through contact-tracing case questionnaires, which they then analyzed using two statistical models.

The models were adjusted for various biases common to virus outbreaks, such as changes in infection rates over time, that would otherwise affect the results.

The mean incubation period was estimated to be 7.6 days in one model and 7.8 days in the other model, while the mean serial interval estimated was 8 days in one model and 9.5 days in the other.

For both models, the median serial interval was between 0.3 and 1.7 days shorter than the median incubation period, indicating that considerable transmission is occurring prior to the onset or detection of symptoms.

Four days was the maximum time transmission was detected before symptoms appeared

Analysis of individual patient-level data, collected from a subset of patients with more detailed information, seemed to confirm this explanation, with 10 of 13 case contact patient pairs reporting presymptomatic transmission. Four days was the maximum time transmission was detected before symptoms appeared.

Based on these results, the researchers say that an isolation period of 16 to 23 days would be required to detect 95% of people with a possible infection.

Although this is an observational study, and the researchers acknowledge several limitations, it is a large study that used robust methods and adjusted for key biases that are present in the data, providing greater confidence in the findings. conclusions.

For this reason, their results have important implications for isolation and contact tracing policies, they say, and that backward contact tracing strategies (tracing who is spreading the disease) should take into account has a presymptomatic infectious period when trying to find the contacts of the confirmed cases.

In a linked editorial, researchers based in the US, UK and Nigeria argue that pre-exposure vaccination and vaccine equity are urgently needed worldwide.

It is likely that the vaccination more profitable to control the consequences of preventable infections, including hospital admissions, loss of income during isolation and long-term complications, they explain.

However, they note that many of the public health measures that have been critical during monkeypox outbreaks in high-income countries remain unavailable in much of Africa.

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