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The resurgence of cases of contagion by the new coronavirus that is causing a second wave greater than the first in Portugal will have started at the end of August, even before the schools are fully operational, and was due to “an excessive resumption of the number of contagious contacts during the summer ”, conclude several researchers in an editorial in which they analyze the role of mathematical models in predicting the future of the pandemic, an article that has just been published in the online edition of Medical Certificate. The first author is Manuel Carmo Gomes, Professor of Epidemiology at the Faculty of Sciences of the University of Lisbon (FCUL) and one of the experts heard by the Government.
In the article, which is signed by other researchers from FCUL, the Department of Mathematics of the University of Coimbra and the University Medical Center of Utrecht (Netherlands), the authors warn that the statistical models they use “predict that we will enter in November with more 3,500 new cases per day ”, although, due to delays in diagnosis and registration, a lower value may be reported at that time. But they emphasize that it is “perfectly possible” that we reach a “peak of more than four thousand cases” daily, “unless in the coming weeks there are drastic changes in the frequency of infections.”
The increase in contagious contacts “is not necessarily homogeneous”, since “most people must have considerably reduced their contacts in relation to the pre-covid era,” they emphasize. The problem lies mainly in localized outbreaks caused by “super-transmission events, such as those that occur at parties or by the entry of the virus into homes”, which can “originate transmission chains that remain undetectable for some time and spread the virus sooner. screening of Public Health teams ”.
Unreliable long-term projections?
But if statistical models are useful to make projections in the short term, in the medium and long term they are already unreliable because they are based solely on the incidence of the disease that occurred in the recent past, the researchers emphasize. Therefore, and to circumvent this problem, “sophisticated” mathematical models are used, which “require knowledge of the biology of the virus, the evolution of the contagion of those infected, the pattern of contacts between people and the way in which they are taken. distance measures and hygiene affect this pattern ”.
The researchers point out, incidentally, that mathematical models have “suggested that only about 20% of those infected will be responsible for 80% of new cases” and that “almost half of infections” are caused by infected people. without symptoms, “which makes it difficult to detect and block transmission chains.”
It was a model of this type that also served to project the impact of the reopening of schools in a work carried out jointly by these researchers and which was presented by Manuel Carmo Gomes, in September, at a meeting with members of the Government, in Porto. Scenarios were simulated with different levels of contact between young people within schools: the same as before the pandemic, 70%, 50% and 30% contacts at that time.
The model allowed to conclude that the reopening of schools had a great propensity to originate a second epidemic wave in Portugal, but that this was not inevitable, at the time the projections were made, in August. However, to avoid the second wave, it would be necessary, simultaneously with the reduction of contacts in schools, a decrease in “contagious contacts” (capable of causing contagion) throughout the community.
“Although in society contagious contacts are reduced to 50% of the pre-covid period, it is necessary that, at a minimum, contacts within schools also reduce to 50%”, they observe. Another important conclusion “is that the return to the level of contacts that existed in pre-covid society (as opposed to the 50% reduction) also triggers a second wave, even if contacts within schools are very low.”
