The serial interval and incubation period for the onset of symptoms and skin rashes for monkeypox virus infection during the current US outbreak

The serial interval and incubation period for the onset of symptoms and skin rashes for monkeypox virus infection during the current US outbreak

In a recently published study medRxiv * Researchers estimated the serial interval and incubation period of monkeypox virus (MPXV) infections for the 2022 disease outbreak in the United States (US).

Lernen: Schätzungen der seriellen Intervalle und Inkubationszeit der Affenpockenvirusinfektion in 12 US-Gerichtsbarkeiten, Mai – August 2022. Bildnachweis: Marina Demidiuk/Shutterstock

*Important NOTE:medRxiv publishes preliminary scientific reports that have not been peer-reviewed and therefore should not be considered conclusive, guide clinical practice/health-related behavior, or treated as established information.

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The US Centers for Disease Control and Prevention (CDC) data showed 75,568 confirmed MPXV cases and 34 deaths in 109 endemic and non-endemic locations as of October 25, 2022. In the 2022 MPXV outbreak, transmission occurred primarily through sexual or close contact with the infectious rash, scabs, or body fluids.

Subsequently, several patients in 12 states in the US reported having had many anonymous sex partners or attending festivals three weeks before symptoms appeared. It complicated efforts to identify primary and secondary case pairs. Furthermore, it complicated the determination of the serial interval for the current MPXV outbreak, a critical determinant of the course of a disease outbreak, and the impact of interventions such as vaccines and drugs. In particular, MPXV symptoms typically manifest within three weeks of exposure and may include fever, headache, chills, swollen lymph nodes, and fatigue. A rash usually develops within one to four days after symptoms appear.

The serial interval is defined as the time elapsed from the onset of symptoms in the primary case to the onset in the secondary case. It determines the effective reproduction number (Rt) and predicts the occurrence of an infectious pathogen.

About studying

In the present study, researchers compiled symptom and rash onset data for primary and secondary case pairs and their type of contact from 12 state and local health departments in the United States. They used Bayesian methods to estimate the serial interval distribution for known primary and secondary case pairs for both symptom onset and rash. They chose the model with the lowest Akaike Information Criterion (AIC) value.

Next, the team estimated the serial interval for symptom onset and rash onset using 50,000 Markov Chain Monte Carlo (MCMC) samples and a burn-in of 10,000 samples. They evaluated the convergence of MCMC samples using Gelman-Rubin statistics. The researchers also included nine and four cases from the District of Columbia and Florida, respectively, in the current study's MPXV incubation period analysis.

Study results

A total of 57 pairs of cases met the study inclusion criteria, of which 40 had rash in primary and secondary cases. The serial interval for symptom onset ranged from two to 25 days for the index MPXV cases, in part due to variability in the type and intensity of contact. The estimated overall serial interval for onset of symptoms and rashes was 8.5 and seven days, respectively.

The authors found that 10/14 new cases (reported from the US alone) were exposed to MPXV in a single day. The average time of MPXV incubation from exposure to symptom onset was 5.6 days, while from exposure to rash onset was 7.5 days in 36 and 35 cases, respectively. Interestingly, in secondary cases there was a statistically significant association between serial intervals and incubation times.

Conclusions

The serial MPXV interval of 8.5 days for symptom onset in the current study was consistent with the estimate of 9.8 days from a study conducted in the United Kingdom after right truncation correction. However, it was much shorter than the 12.5 day generation time reported by a study conducted in Italy. According to the authors, no previous studies have documented serial MPXV spike intervals. They reported a serial interval of seven days for rash onset, which was clearly longer than the rash incubation period of 5.6 days. It suggested that MPXV transmission also occurred before the rash.

In summary, the study estimated the serial interval containing estimates of skin rash occurrence for the current US MPXV outbreak; In addition, it demonstrated its reliability to estimates based on initial disease symptoms.

*Important NOTE:medRxiv publishes preliminary scientific reports that have not been peer-reviewed and therefore should not be considered conclusive, guide clinical practice/health behavior, or be treated as established information.

Reference:

• Vorläufiger wissenschaftlicher Bericht.
  Madewell, Z. et al. (2022) „Serienintervall- und Inkubationszeitschätzungen der Affenpockenvirusinfektion in 12 US-Gerichtsbarkeiten, Mai – August 2022“. medRxiv. doi: 10.1101/2022.10.26.22281516. https://www.medrxiv.org/content/10.1101/2022.10.26.22281516v1

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