The US Centers for Disease Control and Prevention (CDC) on Wednesday reported 186,101 cases of coronavirus, an increase of 22,562 cases from its previous count, and said that the number of deaths had risen by 743 to 3,603.
The CDC reported its tally of cases of the respiratory illness known as COVID-19, caused by a new coronavirus, as of 4 p.m. ET on March 31 compared to its count a day ago.
The CDC figures do not necessarily reflect cases reported by individual states.
In a study, the US CDC also said people infected with the novel coronavirus can transmit the infection one-to-three days before symptoms start to appear.
The study, which underscored the importance of physical distancing to fight the coronavirus outbreak, looked at 243 cases of COVID-19 reported in Singapore between January 23 and March 16.
It identified seven "clusters" where pre-symptomatic transmission was likely, and in four such groups, where the date of exposure could be determined, pre-symptomatic transmission occurred one-to-three days before symptoms appeared in the source patient.
Of the cases in Singapore, 157 were locally acquired and 10 of these were likely transmitted before symptoms started to show.
The fast-spreading virus has claimed over 43,000 lives globally.
The findings suggest that it might not be enough for people showing symptoms to limit their contact to control the pandemic, the researchers wrote in the CDC's Morbidity and Mortality Weekly Report, published online on Wednesday.
The findings increase the challenges of containment measures, the researchers wrote, but said the magnitude of the impact depends on the extent and duration of transmissibility while a patient is pre-symptomatic and that has so far not been clearly established.
The study authors suggested that public health officials conducting contact tracing should strongly consider including a period before symptom onset to account for the possibility of this type of transmission.
Transmissions might take place through respiratory droplets or even speech and other vocal activities such as singing, with the rate of emission corresponding to voice loudness, according to the researchers.
While the cases were carefully investigated, the CDC said it was possible an unknown source might have caused the clusters.
And, the findings could also be affected by incorrect reporting of cases, especially if people had only mild symptoms.
With the focus currently on testing only people who show symptoms, the researchers expect asymptomatic illness to be under-detected.
Still, the CDC data adds to previous reports of individual cases in China that indicated the virus could spread before symptoms start to appear.