Clinico-epidemiological assessment of COVID-19 reinfections during Delta wave, Kerala, India, 2021

Respiratory Diseases

In 2021, the coastal state of Kerala had high test positivity, whilst COVID-19 cases decreased in other parts of India. Reinfections are an important contributor to persistent cases in the context of new variants. In our study we assessed the clinico-epidemiology of COVID-19 reinfections in Kerala.
We conducted a survey in two high burden districts of Kerala (sample size 360). We selected cases by systematic random sampling from the available line-list of reinfections. Reinfection was defined as two COVID-19 positive tests (RT-PCR or Rapid Antigen Test) at least 102 days apart, with the second positive test between May 31- September 30, 2021 (Delta wave period) in a resident of Kozhikode or Ernakulam. Cases were interviewed for occupation, vaccination, symptoms, and hospitalization history.
Of the 295 interviewed cases (response rate=82%, median age was 42 years [17-80]), 35% worked in the informal sector, and 24% reported a co-morbidity (hypertension and diabetes). Among cases, 24% were unvaccinated during reinfection. The median days between second dose of vaccination and reinfection was 111 days [26-246] and between episodes of infection 172 days [97-482]. During both infections, 52% were symptomatic. Hospitalization rate was 7.8% during the first episode and 3% during reinfection (P value=0.06). Oxygen therapy was required in 2.3% during first episode and 1.3% cases during reinfection (P value=0.56). Among reinfections, age >50 years (POR 4.6, 95% Confidence Interval (CI)= 1.5-14.5) and comorbidities (POR 5.7, 95% CI= 1.8-18) were risk factors for hospitalization.
The majority of reinfections were among adults from the informal sector. One quarter were unvaccinated. The infection severity decreased during reinfection. Elderly and co-morbid were at higher risk of hospitalization. We recommended scaling-up of vaccination among informal sector workers, prioritizing booster doses of COVID-19 vaccine in elderly and co-morbid after three months of last vaccination and six months of last infection.

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