Only 1.2% of population of largest city in north of São Paulo State have been infected by SARS-CoV-2
May 27, 2020
By Karina Toledo | Agência FAPESP – In Brazil, preliminary results from a study coordinated by researchers affiliated with the University of São Paulo (USP) suggest that only 1.2% of the 700,000 inhabitants of Ribeirão Preto – the most important city in the north of the state of São Paulo – have been infected so far by the novel coronavirus SARS-CoV-2.
“These results show that the contagion rate is still low in the city and hence that a large proportion of the population is still susceptible to the virus. This reinforces the need to maintain social isolation until other measures can be taken to brake the spread of the disease,” said Benedito Maciel, head of the teaching hospital (Hospital das Clínicas) run by the university’s Ribeirão Preto Medical School (FMRP-USP) and one of the designers of the study, which was conducted in partnership with the city’s health department.
The estimate in the study is based on an analysis of blood samples and nasal secretions from 700 citizens collected in all neighborhoods of the city in the first three days of May. Fifth- and sixth-year medical students took part in the fieldwork. Donors were selected by researchers in the Department of Social Medicine using census-based criteria so as to obtain a representative cross-section of the population.
The blood and mucus samples were tested for SARS-CoV-2 using RT-PCR, a molecular diagnostic technique that detects viral genetic material while the infection is acute. The samples were also analyzed using a rapid serological test that detects antibodies around ten days after exposure.
“Only ten samples returned positive results in the serological test and only one in the molecular test. The fieldwork team went back to the home of the person who tested positive by RT-PCR [meaning the person still had the virus in their organism] to test four other household members. Three had also been infected,” said Rodrigo Calado, a professor in the Department of Medical Imaging, Hematology and Clinical Oncology at FMRP-USP and a collaborator on the project. “In this case it was possible to isolate the people who were infected and prevent them from spreading the virus. This is an important active surveillance measure.”
According to Calado, all the samples that tested positive were from people aged between 20 and 50. This is not considered the highest-risk age group for COVID-19. None was hospitalized, and only some displayed flu-like symptoms in the acute phase.
In his opinion the low contagion rate shows that the quarantine starting on March 20 succeeded in containing the spread of COVID-19 in Ribeirão Preto. “For now the situation in local hospitals is under control,” Calado said. “It’s important to keep track of the numbers so as to be able to plan next steps, which may entail expanding the healthcare infrastructure, maintaining or relaxing the quarantine, or taking other protective measures, such as requiring people to wear masks, for example.”
With this in mind, the project team are planning another round of sample collection in mid-June. All subjects who tested negative in the first round will be re-visited by the medical students. “We want to see how the contagion rate changes after a six-week interval to get an idea of the dynamics of COVID-19 in the city,” said Calado, who is also affiliated with the Center for Cell-Based Therapy (CTC), a Research, Innovation and Dissemination Center (RIDC) funded by FAPESP and hosted by FMRP-USP.
The priority in this kind of initiative, Calado explained, is offering suitable medical care to infected subjects and their contacts, and at the same time interrupting transmission. “An outpatient clinic was set up for convalescing patients. We’ll be monitoring them to look for possible long-term consequences of the disease,” he said.
According to Maciel, the project team would like to conduct more rounds of collection and even test people in other cities of the region. “The problem unfortunately is that rapid tests are expensive. They cost between BRL 120 and BRL 150 each [now about USD 20-27], making expansion of the project a financial impossibility,” he said.
The rapid tests used in the first round, he explained, were supplied by the city government, which received them from the federal health ministry. Funding was also provided by Hospital das Clínicas, the Ribeirão Preto Blood Center, and private donors.
“Researchers at USP are developing a serological test based on ELISA [enzyme-linked immunosorbent assay, which has to be performed in a laboratory]. This will be more affordable,” Maciel said.
The technical committee for the project includes researchers in the Departments of Social Medicine and Clinical Medicine, as well as physicians on the staff of Hospital das Clínicas. Ribeirão Preto had 794 confirmed cases of COVID-19 and 17 deaths from the disease on May 24.
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