Seroprevalence of chikungunya virus in Colombo, Sri Lanka before the 2025 outbreak and implications for population susceptibility.

Following a large chikungunya outbreak during 2006-2008, Sri Lanka did not report any outbreaks for a 16-year period until end of 2008, possibly due to population immunity. Therefore, understanding baseline immunity prior to outbreaks is crucial to inform implementation of vaccine strategies. We assessed the age-stratified seroprevalence for chikungunya in an urban (n=816) and a semi-urban (n=380) community in Colombo, Sri Lanka, from September to November 2024, prior to the commencement of the large chikungunya outbreak, in December 2024. Socio-demographic, socio-economic and clinical data were collected and chikungunya specific IgG measured in serum samples. Of 1196 participants, 410 (34·3%) were chikungunya IgG seropositive. Seroprevalence was significantly higher in urban populations compared with semi-urban populations (39·6% vs 22·9%; p<0·001) and increased significantly with age in urban areas but not in semi-urban areas. Living in an urban area was the strongest independent risk factor of chikungunya seropositivity (aOR 7·48, 95% CI 4·05-13·81; p<0·001), consistent with the higher population density, poor housing conditions and overcrowding observed in that setting. The use of mosquito nets was independently associated with reduced risk of seropositivity (aOR 0·50, 95% CI 0·27-0·93; p=0·029). Almost no individuals aged <16 years had evidence of prior infection (0.55%), indicating minimal transmission in the preceding 16 years. In the urban cohort, seropositivity was significantly associated with diabetes, central obesity, overweight, and hypertension. There appears to have been minimal chikungunya transmission in the 16 years preceding the 2024 outbreak, with a large population susceptible to chikungunya. Higher seroprevalence in urban populations highlights the role of population density, overcrowding, and housing conditions as key drivers of transmission.