Data were summarized as percentages and medians (ranges); 95% confidence intervals were calculated via the binomial method using Free Statistics Software (version 4.0).
One hundred and fifty asymptomatic visitors were screened for COVID-19. There was close contact between them before and during hospitalization.
One possibility is that transmission happened beforehand, since longer incubation periods and transmission from asymptomatic individuals have been reported (1).
Although it is not possible to completely exclude transmissions related to other patients or hospital staff (1) (2) (3) (4) , no other contacts developed symptoms before or afterwards.
Healthcare-related outbreaks of COVID-19 have been reported (2,3) mostly when institutions had not implemented adequate infection control measures (5) . A recent large US study showed a low incidence of hospital-acquired COVID-19 (5). However, J o u r n a l P r e -p r o o f nosocomial infections still occur, and may be closely related to asymptomatic and presymptomatic individuals' potential of transmission (4) .
While implementation of universal masking does reduce transmissions (4-6), they might also induce a false perception of protection that could lead to neglect of other important protective measures (6) . The risk of SARS-CoV-2 introduction into facilities by visitors increases as community transmission is widespread (7), therefore occurrence of outbreaks could be mitigated with restriction of time and physical contact during visitation periods, and reassurance of adequate and frequent hand hygiene, as well as personal protective equipment, when necessary. Besides, active screening for visitors might need to be considered as part of infection control protocols (7).
In summary, surveillance of asymptomatic visitors with a rigorous implementation of protective measures during visitation periods may need to be included in infection control practices in order to reduce transmissions in healthcare settings.
This work received support from the Brazilian Coordination for the Improvement of Higher Education Personnel (CAPES) and the National Council 