Maíra Menezes (IOC/Fiocruz)
A research led by the Oswaldo Cruz Institute (IOC/Fiocruz) points out that the circulation of the novel coronavirus (SARS-CoV-2) started up to four weeks before the first cases were registered in countries in Europe and the Americas. The study, which uses a statistical methodology of inference from death records, indicates that, while countries were monitoring travelers and confirming the first imported Covid-19 cases, community transmission of the disease was already underway.
According to the paper, published in the journal “Memories of the Oswaldo Cruz Institute”, the novel coronavirus started spreading in Brazil around the first week of February. That is, more than 20 days before the first case was diagnosed in a traveler who returned from Italy to São Paulo, on February 26th, and almost 40 days before the first official confirmation of community transmission, on March 13th. In Europe, the circulation of the disease started around mid-January in Italy and between late January and early February, in Belgium, France, Germany, the Netherlands, Spain and the United Kingdom. Early February was also the period when spread started in New York City, in the United States, according to the study.
The research is the first one to point out the period when community transmission started in Brazil and reinforces preliminary evidence of research conducted in Europe based on genetic analysis. It also corroborates findings from studies carried out in the United States, which indicated that the beginning of the viral spread in New York City occurred between January 29th and February 26th.
As in Brazil, Italy, the Netherlands and the United States, community spread had been occurring for two to four weeks when the first imported cases of SARS-CoV-2 were identified by confirmation through laboratory tests carried out among travelers. In the rest of the countries, the first official records of infection in travelers took place a few days before or after the start of local transmission as estimated in the research.
The authors emphasize that, in all the countries under analysis, the circulation of Covid-19 started before the implementation of control measures, such as air travel restrictions and social distancing. “This quite long period of hidden community transmission draws attention to the great challenge of tracking the spread of the novel coronavirus and indicates that control measures must be adopted at least as soon as the first imported cases are detected in a new geographic region,” says the researcher from the Laboratory of AIDS and Molecular Immunology at IOC/Fiocruz, Gonzalo Bello, who is coordinating the research.
Although community transmission has started at very close moments in several countries, the spread of the epidemic in each location seems to have followed its own dynamics. “It is very likely that the dynamics of the epidemic's expansion has been defined by local factors, such as environmental characteristics of temperature, precipitation and air pollution, population density and demography,” adds researcher from the Gonçalo Moniz Institute (Fiocruz-Bahia), Tiago Graf.
Besides helping to clarify when the local transmission of SARS-CoV-2 started in the countries studied, the authors emphasize that the results obtained reinforce the importance of implementing permanent molecular surveillance actions, since the novel coronavirus may circulate again and cause outbreaks over the next few years. “Intense virological surveillance is essential to detect the potential reemergence of the virus on an earlier basis, informing contact-tracking systems and providing evidence to take the appropriate control measures,” adds Gonzalo.
The study has been carried out by the Laboratory of AIDS and Molecular Immunology at IOC/Fiocruz in partnership with Fiocruz-Bahia, Federal University of Espírito Santo (Ufes) and University of the Republic (Udelar), in Uruguay. The results have been published in the “Fast Track” section of the scientific journal “Memories of the Oswaldo Cruz Institute”, which allows for accelerated dissemination of research related to the pandemic.
Gonzalo and the entire team involved in the work have wide experience in research applied to understanding the onset of outbreaks and epidemics, having conducted studies with techniques based on genetic analysis for HIV, hepatitis, dengue, Zika and yellow fever, among other diseases. “This type of analysis is a valuable contribution of Science to Public Health, as it helps to elucidate a time gap immediately prior to the initial detection of cases,” says the researcher.
For this type of investigation, it is essential to have a representative volume of virus genomes found in samples taken from patients. “However, the short time elapsed since the beginning of the epidemic, combined with the limited amount of SARS-CoV-2 genomes available, makes it very difficult to apply molecular genomics in order to infer the onset of local transmission in most countries,” comments UFES researcher, Edson Delatorre.
Therefore, in order to estimate the period when community viral transmission of the novel coronavirus began, the researchers have developed a new method. The scientists' starting point has been one of Covid-19's most tragic and striking traits: the exponential growth in the number of deaths in the first weeks of outbreak. Since the lack of diagnostic tests and the large percentage of asymptomatic infections make counting the cases of the disease difficult, death records are considered the most reliable information on the progress of the epidemic and can be used as a “delayed” tracker, which allows looking at the course of the disease in retrospect.
Considering that the average time between infection and death by Covid-19 is about three weeks and the mortality rate from the disease is approximately 1%, scientists have applied a statistical method to infer the moment when the epidemic started based on the cumulative number of deaths in the first weeks of an outbreak in each country. “Looking at the two countries where there is already a large number of sequenced genomes – China and the United States – we have found that the estimate obtained from the number of deaths is similar to that obtained from genetic analysis, validating the new approach,” says the Udelar researcher, Daiana Mir.