We used the search terms “Basic reproduction number” or “R₀” AND “chikungunya” to identify published articles from Google Scholar and PubMed. We identified 11 articles describing estimated R₀ of CHIKV from outbreak data during 2000–2019. We found 5 articles on outbreaks in Africa, all on La Réunion (3,5–8); 1 on an outbreak in Cambodia (1); 2 on outbreaks in Italy (9,10); and 3 on outbreaks in the Americas (11,12; N. Báez-Hernández et al. unpub. data, https://www.biorxiv.org/content/10.1101/122556v1).

The authors estimated R₀ by using mathematical (compartmental) models fitted with respective outbreak data (1,3,5–12). We considered the estimated values comparable and extracted the R₀ from each. We then estimated the weighted mean R₀ of CHIKV based on outbreak size, such as number of reported cases included in the estimation of R₀ in the original article, and further estimated the mean R₀ for different mosquito vectors and E1-A226V gene mutations.

The largest CHIKV outbreak occurred on La Réunion and affected 266,000 of the 785,000 inhabitants (3). Several models with differing levels of data estimated the R₀ of the La Réunion outbreaks between 0.89 and 4.1 (3,5–7). The R₀ also was estimated from CHIKV outbreaks in Italy in 2007 (10) and 2017 (9), Cambodia in 2012 (1), Venezuela in 2014 (11), Colombia in 2015 (12), and Mexico in 2015 (N. Báez-Hernández et al. unpub. data, https://www.biorxiv.org/content/10.1101/122556v1) (Table).

We estimated the weighted mean R₀ of CHIKV to be 3.4 (95% CI 2.4–4.2). We analyzed the data and estimated the R₀ for Ae. aegypti and Ae. albopictus mosquitoes separately for outbreaks in which the R₀ of CHIKV was described for each species. We estimated the R₀ to be 4.1 (95% CI 1.5–6.6) for Ae. aegypti mosquitoes and 2.8 (95% CI 1.8–3.8) for Ae. albopictus mosquitoes. Although the difference is not statistically significant (p = 0.12), we expected a lower R₀ for outbreaks involving Ae. albopictus mosquitoes because this species also feeds on animals, which might have reduced the attack rate on humans and transmission across the population. However, outbreaks associated with Ae. albopictus mosquitoes can be prolonged and the outbreak response can have economic consequences. We estimated the R₀ to be 3.5 (95% CI 1.9–4.9) during outbreaks involving the E1-A226V mutation, which is higher than R₀ of 2.1 from the 2017 outbreak in Italy that did not have the gene mutation.

CHIKV infections among humans can have severe health consequences, despite the low case fatality rate. CHIKV infection has 3 stages: acute, postacute, and chronic. The acute phase usually lasts for 1–3 weeks and is characterized by fever, intense myalgia, arthralgia, and symmetric joint pain in both legs that can limit even the simplest daily activities. The postacute stage usually lasts 1–3 months after the acute phase and is characterized by persistent inflammatory arthralgia, arthritis, tenosynovitis, and bursitis. The chronic stage starts after 3 months and can last for months to years after acute infection (2).

In a study in Brazil, >68% of persons with CHIKV remained chronically infected for up to 1 year (13). On Réunion Island, a small group of patients had clinical signs for 6 years. Although the reason for persistence is unclear, it might be strain related and associated with the E1-A226V mutation. Therefore, despite being less severe and causing fewer deaths than other mosquitoborne diseases, CHIKV can have lingering physical and psychological consequences for those affected. Infected persons also can experience economic consequences because they might not be able to work for several weeks or more.

R₀ does not remain constant. For arboviruses, R₀ can vary based on the density of hosts and vectors; mosquito species, survival, and biting rate; and vector competence and capacity, all of which can depend on environmental and microclimatic factors. Further, the vector competence of Ae. aegypti mosquitoes for CHIKV might be different from that for Ae. albopictus mosquitoes, which could influence outbreak dynamics. For example, 1 study reported the transmission efficiency of Ae. albopictus mosquitoes as 97% and of Ae. aegypti mosquitoes as 83% (14).

The outbreaks included in our study occurred in tropical and subtropical countries and in the more temperate climate of Italy. We did not consider climatic conditions during reported outbreaks, which might play a role in determining the size and R₀ of CHIKV outbreaks. We also did not consider the variation of data quality in published articles, except for the outbreak size, which might affect estimated R₀. However, defining adjustments for data quality would have been difficult and might have introduced unwanted bias.

We found the overall mean R₀ for CHIKV was 3.4 (95% CI 2.4–4.2). Our estimated R₀ of 4.1 (95% CI 1.5–6.6) for Ae. aegypti mosquitoes suggests CHIKV could spread rapidly and cause high disease incidence in urban areas, where this species thrives. Our estimated CHIKV R₀ for Ae. albopictus mosquitoes of 2.8 (95% CI 1.5–6.6) was lower than for Ae. aegypti mosquitoes. In rural areas, where Ae. albopictus mosquitoes are more prevalent, sylvatic cycles, maintenance of biodiversity including natural mosquito populations, and presence of hosts other than humans might reduce the effects of an outbreak. Early interventions targeting Aedes mosquitoes will be vital to controlling CHIKV outbreaks.