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New study estimates monkeypox incubation, generation times and R0


The first monkeypox (MPX) infection that initiated the current global outbreak was identified in Europe in early May 2022. To date, over 59,000 MPX cases have been reported worldwide.

Study: Early Estimates of Monkeypox Incubation Period, Generation Time, and Reproduction Number, Italy, May–June 2022. Image Credit: Cryptographer / Shutterstock.com

Background

Soon after the initial MPX case was identified in Europe, the European Centre for Disease Prevention and Control and the World Health Organization (WHO) requested their member states to accurately report both suspected and confirmed MPX cases. Throughout this outbreak, most of the MPX cases have been diagnosed among men who have sex with men (MSM).

Following initial outbreak reports in Italy, several protective measures were recommended by the Ministry of Health of Italy to control the spread of the MPX virus. These included implementing quarantine measures, limiting contact and exposure for healthcare workers, monitoring symptom onset, and contact tracing.

A new Emerging Infectious Diseases journal study discusses the characteristics of the first 255 reverse transcription polymerase chain reaction (RT-PCR)-confirmed MPX cases in Italy in 2022.

About the study

The current study only included MPX cases that were confirmed through RT-PCR. In addition to their MPX infection status, the researchers also obtained information on various patient characteristics such as sex, age, presence of rash or other symptoms, the earliest date of symptom onset, travel abroad, and exposure modality.

The incubation period was determined for 30 cases with known dates and periods of symptom exposure. The generation time was determined using 16 infector-infectee pairs identified through contact tracing efforts. The net reproduction number was calculated using the generation time.

Study findings

Only two of the 255 MPX cases in the current study were not identified in men, with 95% of the MPX cases being identified in MSM. The median age of the study participants was 37 years.

Out of 184 cases where additional information was available, 139 cases reported rashes in the perianal or genital area. Moreover, 151 of 222 cases reported fever.

Of the 228 cases whose travel information was available, 37% reported previous travel abroad. Among these individuals, 25 reported recent travel to the Canary Islands, while one traveled to West Africa.

The mean incubation period was 9.1 days, while the generation time was 12.5 days. Moreover, the net reproduction number was 2.43 during the first week of June and subsequently decreased after June 12, 2022.

The results of the current study provide important information regarding the early stages of the current MPX outbreak in Italy. These observations also elucidate the duration for which symptoms must be monitored, infected people must be isolated, and contact tracing is necessitated. In addition, the net reproduction number is also a valid parameter to monitor the gradual spread of the disease.

Limitations

The relatively small size of the current study limits the generalizability of these findings. Furthermore, selection bias regarding infector-infectee pairs and recall bias concerning exposure dates were also evident. An additional limitation of the current study is that ‘superspreading’ events might have inflated the observed reproduction number.

Journal reference:

  • Guzzetta, G., Mammone, A., Ferraro, F., et al. (2022). Early Estimates of Monkeypox Incubation Period, Generation Time, and Reproduction Number, Italy, May–June 2022. Emerging Infectious Diseases. doi:10.3201/eid2810.221126.



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