Transmission dynamics of monkeypox in the United Kingdom: contact tracing study

Abstract

Objective To analyse the transmission dynamics of the monkeypox outbreak in the UK, declared a Public Health Emergency of International Concern in July 2022. Design Contact tracing study, linking data on case-contact pairs and on probable exposure dates. Setting Case questionnaires from the UK Health Security Agency (UKHSA), United Kingdom. Participants 2746 people with polymerase chain reaction confirmed monkeypox virus in the UK between 6 May and 1 August 2022. Main outcome measures The incubation period and serial interval of a monkeypox infection using two bayesian time delay models—one corrected for interval censoring (ICC—interval censoring corrected) and one corrected for interval censoring, right truncation, and epidemic phase bias (ICRTC—interval censoring right truncation corrected). Growth rates of cases by reporting date, when monkeypox virus was confirmed and reported to UKHSA, were estimated using generalised additive models. Results The mean age of participants was 37.8 years and 95% reported being gay, bisexual, and other men who have sex with men (1160 out of 1213 reporting). The mean incubation period was estimated to be 7.6 days (95% credible interval 6.5 to 9.9) using the ICC model and 7.8 days (6.6 to 9.2) using the ICRTC model. The estimated mean serial interval was 8.0 days (95% credible interval 6.5 to 9.8) using the ICC model and 9.5 days (7.4 to 12.3) using the ICRTC model. Although the mean serial interval was longer than the incubation period for both models, short serial intervals were more common than short incubation periods, with the 25th centile and the median of the serial interval shorter than the incubation period. For the ICC and ICRTC models, the corresponding estimates ranged from 1.8 days (95% credible interval 1.5 to 1.8) to 1.6 days (1.4 to 1.6) shorter at the 25th centile and 1.6 days (1.5 to 1.7) to 0.8 days (0.3 to 1.2) shorter at the median. 10 out of 13 linked patients had documented pre-symptomatic transmission. Doubling times of cases declined from 9.07 days (95% confidence interval 12.63 to 7.08) on the 6 May, when the first case of monkeypox was reported in the UK, to a halving time of 29 days (95% confidence interval 38.02 to 23.44) on 1 August. Conclusions Analysis of the instantaneous growth rate of monkeypox incidence indicates that the epidemic peaked in the UK as of 9 July and then started to decline. Short serial intervals were more common than short incubation periods suggesting considerable pre-symptomatic transmission, which was validated through linked patient level records. For patients who could be linked through personally identifiable data, four days was the maximum time that transmission was detected before symptoms manifested. An isolation period of 16 to 23 days would be required to detect 95% of people with a potential infection. The 95th centile of the serial interval was between 23 and 41 days, suggesting long infectious periods.

Monkeypox

“…However, seroepidemiological studies in Africa, 3 , 97 and retrospective PCR detection in male sexual health clinic attendees in France 98 and Belgium, 99 suggest that some patients could have an asymptomatic infection, and modelling studies suggest that in the 2022 outbreak approximately half of the transmissions occurred in the pre-symptomatic phase. 100 Further research is needed to determine the potential for transmission from individuals with an asymptomatic infection.…”

The dynamics of monkeypox transmission

“…In a linked paper, Ward and colleagues (doi:10.1136/bmj-2022-073153) used routine case questionnaires and contact tracing data to estimate two important characteristics of the monkeypox outbreak in the UK: the serial interval and incubation period 3. They found that shorter serial intervals are more common than shorter incubation periods.…”

Time scales of human mpox transmission in the Netherlands

“…Knowledge of the serial interval is key, as it informs on the reproduction number and the required intensity of control measures to stop an outbreak. Current estimates of the mean serial interval of monkeypox vary with a recent study estimating the mean serial interval as 5.6 days 4 , while estimates have been reported of 8.5 days in the US 5 , 9.5 days in UK 6 , and 12.5 days in Italy 7 . There is no general consensus on an estimate of mean serial interval for the current monkeypox outbreak, largely due to the limited availability of reliable data.…”