Abstract
Background: Little is known about the characteristics of those who
transmit SARS-CoV-2 infection vs those who do not, but this information
could inform disease control policies. This study described the features
of clusters in the first wave of COVID-19 in Shanghai and compared
contagiousness by clinical and health care risk factors. Methods: In
this retrospective cohort study of cases in Shanghai in January and
February 2020, cases with successive generations were considered to be
“contagious.” Characteristics of contagious and non-contagious cases
were compared in log-binomial models that also adjusted for age and sex.
Results: Between January 21 and February 17, 2020, 333 cases of COVID-19
were reported in Shanghai across 28 known infection chains.
Contagiousness was higher among cases with a sore throat (risk ratio
[RR]: 3.41, 95% CI: 1.59, 7.35, P=0.0051), and those with heart
disease (RR: 2.06, 95% CI: 0.72, 5.90). Delays in diagnosis were also
associated with higher risk of contagiousness. Having ≥2 medical visits
before diagnosis was associated with 4.46 times higher risk of
contagiousness (95% CI: 2.03, 9.83, P=0.0002), and there was a
non-significant increase in risk with increasing numbers of days between
disease onset and isolation (for each day, RR: 1.08, 95% CI: 1.01,
1.16, P=0.1734). Conclusions: Individuals with mild COVID-19 symptoms in
the upper respiratory tract may still be contagious, and such
individuals should be prioritized for early diagnosis and isolation to
limit further chains of transmission.