HealthDay News — Individuals with COVID-19 are most infectious in the two days before and three days after symptom onset, according to a study published online Aug. 23 in JAMA Internal Medicine.

Yang Ge, from the University of Georgia in Athens, and colleagues examined the correlation between timing of exposure and development of disease among close contacts of index patients with COVID-19 in a population-based cohort study involving 730 index patients who received a diagnosis of COVID-19 from Jan. 8 to July 30, 2020. A total of 8,852 close contacts of the index patients were visited and evaluated.

The researchers found that the contacts were at highest risk for COVID-19 if they were exposed between two days before and three days after the index patient’s symptom onset, peaking at day 0 (adjusted relative risk, 1.3). The risk for COVID-19 among contacts was higher when they were exposed to index patients with mild and moderate cases of COVID-19 compared with being exposed to an asymptomatic index patient (adjusted relative risks, 4.0 and 4.3, respectively). Infected contacts were less likely to be asymptomatic as index case severity increased (adjusted relative risks, 0.3 and 0.3 for exposure to patients with mild and moderate COVID-19, respectively).


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“Our results suggest that the timing of exposure relative to primary-case symptoms is important for transmission, and this understanding provides further evidence that rapid testing and quarantine after someone is feeling sick is a critical step to control the epidemic,” a coauthor said in a statement.

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