Beneath the Tip of an Iceberg: Estimating Mpox Case Numbers in Indonesia
As of August 17, 2024, Indonesia's Ministry of Health (MoH) reported 88 confirmed cases of Monkeypox (Mpox) in the country since 2022. The distribution of cases is as follows: 59 in Jakarta, 13 in West Java, 9 in Banten, 3 in East Java, 3 in Yogyakarta Special Region, and 1 in Riau Islands. Of these cases, 87 have been declared recovered.[1]
Mpox is caused by the monkeypox virus, which is an enveloped, double-stranded DNA virus belonging to the Orthopoxvirus genus within the Poxviridae family. This family also includes variola (the cause of Smallpox), Cowpox, Vaccinia, and other related viruses. JYNNEOS is a two-dose vaccine designed to provide protection against both Mpox and Smallpox. It is essential for individuals to receive both doses of the vaccine to ensure optimal protection against Mpox. The Monkeypox virus has two distinct clades: clade I (with subclades Ia and Ib) and clade II (with subclades IIa and IIb).[2]
Clade I is linked to the recent increase in cases in Central and Eastern Africa and has historically been associated with a higher incidence of severe illness compared to clade II, with a fatality rate of up to 10%. However, recent outbreaks have seen much lower death rates, around 1-3.3%.[3] In contrast, clade II, which caused the global outbreak beginning in 2022, generally leads to milder infections, with over 99.9% of individuals recovering. Clade II is endemic to West Africa.
Both types of the virus primarily spread through close contact with an infected person, such as skin-to-skin contact, kissing, or being in close proximity, which can expose you to respiratory particles. Those with multiple sexual partners are at higher risk. The virus can also be contracted from contaminated objects like clothing or linens, needle injuries in healthcare or community settings, and can be passed to a baby during pregnancy or birth.
Individuals with Mpox are considered contagious until all their lesions have crusted over, the scabs have fallen off, and a new layer of skin has formed underneath. Additionally, all lesions in the eyes and inside the body (including the mouth, throat, eyes, vagina, and anus) must be fully healed, a process that typically takes 2 to 4 weeks.[4]
M Kristi Endah Murni, Director General of Air Transportation, stated that under Circular Letter SE 5 DJPU Year 2024, all international travelers, including airline personnel and passengers flying to Indonesia, must complete the SATUSEHAT Health Pass electronic self-declaration form. This measure was introduced following the World Health Organization's designation of Mpox as a Public Health Emergency of International Concern on August 14, 2024.[5]
Given the level of public health urgency that Mpox represents for Indonesia, I believe it is essential to provide an educated estimate of potential cases in the country, based on the effective reproductive number (Re) of Mpox.
Re represents the expected number of new infections generated by a single infectious individual during their infectious period in a population that includes both susceptible and immune individuals. It is influenced by a range of biological, sociobehavioral, and environmental factors.
On September 14, 2021, during the enforcement of the Public Activity Restriction Policy (PPKM), data from Indonesia estimated the effective reproduction number (Re) at 0.59.[6] This indicates that, on average, a single COVID-19 case would result in fewer than one secondary infection within a population that includes both susceptible and immune individuals. An Re value below 1 suggests that the infection is unlikely to continue spreading. The mathematical formula for Re is as follows:
Re = R0・x
R0 is the basic reproductive number. It represents the expected number of new infections that a single infectious individual will generate during their infectious period in a fully susceptible population. Furthermore, x is the proportion of susceptible individuals in the population. Additionally, R0 can be calculated using the following formula:
R0 = c・p・d
The variable c represents the contact rate, defined as the average number of interactions with an infectious individual per unit of time. A study conducted in Suzhou, China, found that the average number of social contacts per day was 11.51 (SD 5.96).[7] Given the similar population density between Indonesia and China, I assume the contact rate from the Chinese context applies to Indonesia.[8]
p refers to the probability of transmission per contact with an infected person, indicating the likelihood of infection occurring during each interaction. The probability of Mpox transmission per contact ranges between 0.1 and 0.9.[9]Lastly, d represents the duration of infectiousness, or the length of time an individual remains infectious. For Mpox, this period typically lasts between 14 and 28 days.
By using publicly available data, we can conduct a simplified sensitivity analysis to estimate the minimum and maximum R0 for Mpox in Indonesia. At the minimum values (c: 11.51 contacts per day, p: 0.1 transmission probability, d: 14 days), the estimated R0 for Mpox in Indonesia was:
R0 = c・p・d
R0 = 11.51/day・0.1・14 days
R0 = 16.114
At the maximum values (c: 11.51 contacts per day, p: 0.9 transmission probability, d: 28 days), the estimated R0 for Mpox in Indonesia was:
R0 = c・p・d
R0 = 11.51/day・0.9・28 days
R0 = 290.052
Given an estimated 69% varicella vaccination rate,[10] we can proceed to calculate the effective reproduction number (Re) of Mpox cases in Indonesia during that specific time frame. At a minimum, the Re for Mpox in Indonesia could be:
Re = R0・x
Re = 16.114 ・0.21
Re = 3.384
At the maximum values, the Re for Mpox in Indonesia could be:
Re = R0・x
Re = 290.052 ・0.21
Re = 60.911
It can now be estimated that a single confirmed Mpox case in Indonesia in 2022 may have resulted in the transmission of between 4 and 61 secondary cases. When extrapolated by a factor of 88, this suggests that there may have been between 353 and over 5,300 secondary Mpox infections in Indonesia during 2022 alone.
It is, however, important to acknowledge the limitations of these estimates. The calculations rely on several assumptions, such as using contact rate data from Suzhou, China, which may not fully reflect social interaction patterns in Indonesia. Additionally, the probability of transmission per contact and the duration of infectiousness are based on general ranges, which might not account for variations in individual behavior, local public health interventions, or virus characteristics specific to the Indonesian context. Furthermore, the use of a 69% Varicella vaccination rate as a proxy for population immunity could introduce uncertainties, as it may not fully capture the immunity landscape relevant to Mpox. These factors suggest that the actual number of secondary Mpox cases in Indonesia could differ from the estimates presented.
In conclusion, the current assessment of Mpox cases in Indonesia underscores the importance of understanding the dynamics of this virus within the population. While the reported cases by the Ministry of Health provide a starting point, the extrapolation of potential secondary infections reveals a significantly higher burden than initially perceived. This analysis not only highlights the urgency of public health interventions but also emphasizes the need for ongoing surveillance, vaccination efforts, and community education to mitigate the spread of Mpox. As the situation evolves, it is crucial for health authorities and stakeholders to remain vigilant and responsive, adapting strategies to protect the health of the Indonesian population and ensure a robust public health response to emerging infectious diseases.
Footnotes:
[1] Rokom. (2024, August 18). 88 Kasus Konfirmasi Mpox di Indonesia, Seksual Sesama Jenis Jadi Salah Satu Penyebab. Sehat Negeriku. Link
[5] Salsabilla, R. (2024, September 4). Update Terbaru Mpox di RI: Ada Suspek Baru di Bali & Jakarta. CNBC Indonesia. Link
[6] Eqqi, S. (2021, September 20). Effective Reproduction Number 0,59, Infeksi di Masyarakat Turun. Detiknews. Link
[7] Wang, M., Wang, C., Gui, G., Guo, F., Zha, R., & Sun, H. (2024). Social contacts patterns relevant to the transmission of infectious diseases in Suzhou, China following the COVID-19 epidemic. Journal of Health Population and Nutrition, 43(1). https://doi.org/10.1186/s41043-024-00555-x
[8] According to World Development Indicators by the World Bank Group, Indonesia's population density in 2021 was 144.65 individuals per square kilometer, while China's was 150.44 individuals per square kilometer. Source: Link
[9] Omame, A., Han, Q., Iyaniwura, S. A., Ebenezer, A., Bragazzi, N. L., Wang, X., Kong, J. D., & Woldegerima, W. A. (2024b). Understanding the impact of HIV on mpox transmission in an MSM population: a mathematical modeling study. Infectious Disease Modelling, 9(4), 1117–1137. https://doi.org/10.1016/j.idm.2024.05.008
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