Preplanned Studies: The Epidemiological Characteristics of Mpox Cases — China, 2023

In May 2023, the WHO declared the termination of the Public Health Emergency of International Concern (PHEIC) initiated in July 2022 due to the mpox outbreak (1). During the PHEIC, China reported only one imported case in September 2022 (2). Subsequent to this, in June 2023, China reported its first local mpox cases. By December 31, 2023, there were 1,712 confirmed cases across 29 provincial-level administrative divisions (PLADs). Our study used national surveillance data, including all confirmed mpox cases reported following a standardized protocol and unified epidemiological form, to delineate the key epidemiological characteristics of the mpox cases reported in China from June to December 2023, thereby enhancing the understanding of the outbreak’s initial local transmission dynamics.

Case surveillance, diagnosis, contact tracing, and management were conducted in adherence to the Mpox Prevention and Control Protocol. Local CDCs supplied detailed epidemiological data through comprehensive investigations following the identification of suspected or confirmed mpox cases. All case information was collected in line with the “Law of the People’s Republic of China on Prevention and Treatment of Infectious Diseases,” under the provisions for emergency response, thereby exempting the study from requiring ethics approval and participant consent. Additionally, individual data were de-identified to ensure patient privacy and confidentiality.

Descriptive statistics were used to summarize the epidemiological characteristics of the mpox cases. An epidemic curve, constructed from the dates of illness onset and diagnosis, depicted the trend of the epidemic. Demographic and epidemiological attributes of confirmed cases were presented on a monthly basis, using both absolute and relative frequencies. It should be noted that variations in case numbers across different categories may arise from incomplete data.

Among the 29 PLADs that reported confirmed cases of mpox, the highest numbers were observed in Guangdong, Beijing, Zhejiang, Sichuan, and Jiangsu, with counts of 342, 258, 183, 142, and 123 cases, respectively. Together, these regions accounted for 61.21% of all reported cases. Additionally, 24 PLADs (approximately 80%) reported fewer than 100 cases each, and 8 reported fewer than 10 cases.

According to the onset date curve depicted in Figure 1A, the initial case of mpox likely occurred in late May, succeeded by a steady rise in case numbers through the first 20 days of June. From late June through the first week of July, there was a sharp increase in the number of incidences. Concurrently, the geographical spread of the cases widened. The incidence began to diminish in September, stabilizing at relatively low levels through November and December, with an average daily incidence of fewer than three cases. The diagnosis date curve, shown in Figure 1B, mirrored this trend.

Figure 1. 

Epidemiological curves of confirmed mpox cases in China, June to December 2023. (A) Number of cases by date of onset; (B) Number of cases by date of diagnosis.

Among the confirmed cases, 1,702 (99.42%) were male, while 10 (0.58%) were female. The median age of the affected individuals was 31 years, with a range from 15 to 71 years. Notably, 112 (6.54%) cases were individuals born before 1980, who, likely in accordance with the vaccination policies of China at that time, may have received the smallpox vaccine. The predominant demographic, representing 84.17% (1,441/1,712), comprised males aged between 18 and 39 years. Based on their current residential districts, most cases were inferred to reside in urban areas, with only approximately 72 cases residing in towns or rural locations. Regarding occupation, the most frequently reported was “unemployed,” accounting for 39.54% of all cases (677/1,712). This was followed by positions in commercial services (23.01%, 394/1,712), office workers (8.53%, 146/1,712), and laborers (8.29%, 142/1,712). Additionally, there were 56 cases who were students; this group included 6 individuals under the age of 18, composed of 5 males and 1 female (Table 1).

Among the 1,654 male cases for which relevant information was provided, 94.68% (1,566/1,654) were identified as men who have had sex with men (MSM). Of these, 8.81% (138/1,566) reported being married to women. All 10 female cases reported being heterosexual.

Among the cases for which epidemiological data were available, only 4.02% (67/1,666) had traveled outside China within three weeks prior to the onset of illness. Additionally, 42.56% (709/1,666) tested positive for human immunodeficiency virus (HIV). None of the cases reported a history of blood transfusion within the 21 days preceding the onset of their symptoms.

Among the 1,566 cases identified as MSM, 1,419 (90.61%) confirmed engaging in homosexual activities, with each case involving an average of 1.5 partners (as reported by 1,242 cases with available data) in the 21 days preceding symptom onset. The majority of these sexual encounters involved partners who met through social media apps or other online platforms (74.88%, 450/601) or were random encounters in public venues such as bars or bathhouses (8.49%, 51/601). Among the 88 male patients who did not identify as MSM, 17 reported sexual contact with women, and 5 with men, within 21 days before becoming ill. The remaining individuals declined to disclose their sexual activity.

Among the 10 female mpox cases examined, four reported having sexual contact with their male partners, all of whom were confirmed cases and had recently engaged in homosexual activities. Three other cases involved women who had sexual contact with their male partners; among these, two partners developed rashes that, while suggestive of mpox, had not been confirmed by laboratory tests. The third partner denied exhibiting any symptoms associated with mpox. Additionally, two cases occurred in women who were family members of confirmed mpox cases, likely acquiring the infection via general household contact. The final case involved a nurse who contracted the infection through direct exposure while providing medical care to a confirmed mpox patient, representing a probable instance of occupational transmission among healthcare workers.

Among the 1,666 cases for which epidemiological information was available, 99 clusters were identified across 22 PLADs, accounting for 12.85% (214/1,666) of the total cases. These clusters included 85 clusters with two cases, 12 clusters with three cases, and two clusters with four cases. Notably, no instances of third-generation transmission were observed within these 99 clusters. In five of these clusters, it is suspected that the initial case contracted the infection while traveling abroad, subsequently leading to local transmission. However, in the remaining 94 clusters, the definitive sources of infection could not be identified. From June to December, the proportion of cases included in these clusters displayed a decreasing trend, falling from 26.4% in June to 2.8% in November and 4.3% in December.

Among the 1,655 cases for which data were available, 92.93% (1,538/1,655) were diagnosed with mpox upon seeking medical care for their symptoms. An additional 5.26% (87/1,655) were diagnosed as close contacts of confirmed cases during testing initiatives. Moreover, 23 individuals self-reported as potential mpox infections, three cases were identified through active screening surveillance targeting high-risk populations, three cases emerged from health declarations at customs upon entry, and one case was detected during routine physical examinations. Among the 1,537 cases with detailed timelines, the median interval between the onset of symptoms and reporting was 7 days, while the interval between symptom onset and diagnosis was 8 days. These intervals remained relatively consistent from June to December.