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.

Fig 1

Equations

Fig 2

Posterior distributions for mean and standard deviation of the incubation period for monkeypox. Posteriors are reported for the ICC (interval censoring corrected) model and the ICRTC (interval censoring right truncation corrected) model, fit to data from 54 patients using a Weibull distribution

Fig 3

Posterior distributions for mean and standard deviation of the serial interval for monkeypox. Posteriors are reported for the ICC (interval censoring corrected) model and the ICRTC (interval censoring right truncation corrected) model, fit to data from 79 patients using a gamma distribution

Fig 4

Cumulative distribution function of the serial interval and incubation period for monkeypox. Using the ICC (interval censoring corrected) model and the ICRTC (interval censoring right truncation corrected) model, fit to data using a gamma distribution for the serial interval and a Weibull distribution for the incubation period, with 95% credible interval

Fig 5

Onset to exposure, serial interval, and incubation period for 13 primary cases with monkeypox who had matching personal identifiable information or were matched but without such information. The origin is symptom onset date of the primary case, with the x axis showing number of days between subsequent events. Purple arrow indicates number of days between primary onset and secondary exposure. Blue arrow indicates number of days between secondary exposure and secondary onset (incubation period). Red arrow indicates number of days between primary onset and secondary onset (serial interval). *Personal identifiable information not available and therefore data considered less reliable

Fig 6

Doubling and halving times and log scale model fit of case numbers of monkeypox by reporting date. Cases were modelled as a smooth function of time using a generalised additive model with a negative binomial error distribution. Shaded area denotes span of time series where the serial intervals and incubation period distributions were estimated (6 May to 1 August 2022)