In August 2020, when the city of Manaus (AM) saw a sharp drop in Covid-19 cases for three months despite the reopening of schools and trade, part of Brazilian experts hypothesized that the threshold of collective immunity to coronavirus Sars-CoV-2 would have been achieved in the region, although serological tests throughout the north of the country showed a seroprevalence of less than 30%.
The hypothesis gained strength the following month, in September, when researchers from the University of São Paulo (USP) and coworkers published an article using samples from blood banks that, through mathematical modeling, estimated seroprevalence in the capital Amazon to be 66%, that is, it would be close to the threshold calculated at the beginning of the pandemic according to the classic formula used in epidemiology.
When the final version of the study coordinated by Professor Ester Sabino in the journal Science came out in December, 76% of Manauaras already estimated immunity to the new coronavirus. So how do you explain the second wave of cases that led to another collapse of the healthcare system and forced the mayor to declare a state of emergency for the next six months on January 5th?
Sabino estimates that as the virus continues to spread across the country, the number of cases will rise again when people return to normal activities and will continue to increase until it infects around 95% of the population. “There is a misunderstanding of the concept of herd immunity. When the threshold is reached, it does not mean that the disease will go away, but that the cases will not grow as quickly as they did in the first wave. There will hardly be a high point like in April [de 2020] – Unless people have already lost immunity and cases of reinfection are far more common than you think, ”says the researcher.
According to Sabino, the big problem in Manaus is that there are only a few hospitals and the beds in the intensive care unit are quickly used up. “The situation is still worrying. Either the beds in the intensive care unit are triple or it will be necessary to stop the city because today a person with appendicitis can die from a lack of care, ”he says.
Based on data from the Fundação de Medicina Tropical Doutor Heitor Vieira Dourado (AM), infectologist Júlio Croda says 99% of the new cases reported in Manaus are from people who have never had the disease before, i.e. no reinfections.
“In this second wave, most of the patients are from grades A and B who managed to remain isolated during the first wave. The evidence of this is that the private health system was exhausted in front of the public – unlike in April 2020. After the control measures were relaxed, the virus circulated with greater intensity and reached the most affected part of the population. susceptible, ”says Croda, researcher at the Oswaldo Cruz Foundation (Fiocruz) and professor at the Federal University of Mato Grosso do Sul (UFMS).
For the infectologist, the seroprevalence of 76% mentioned in the study published in Science is overestimated. He believes that more than 50% of Manau Macaws have not yet developed immunity to Sars-CoV-2.
“It is important to note that the herd immunity threshold is not a fixed value. It is calculated based on the infection rate [Rt, inicialmente estimado entre 2,5 e 3, ou seja, cada infectado transmite o Sars-CoV-2 para outras duas ou três pessoas]This depends on both the genetics of the virus and the measures taken to contain the spread. Recently, a more transferable variant has emerged in the UK, which affects both the calculation of RT and the threshold of collective immunity, ”explains Croda.
According to the researcher, the care of the population regardless of the public purse – such as wearing masks, hand hygiene and social distance – helps to lower the infection rate, which also lowers the threshold of collective immunity. “This was probably the reason for the decline in the number of cases observed in mid-2020. The moment the control measures by the population and the authorities relaxed, the threshold returned to a level of almost 70%.” , rated.
Paulo Lotufo, an epidemiologist at the University of São Paulo Faculty of Medicine (FM-USP), considers it a “mistake to rely too much on this type of indicator”. “The big problem is that to calculate the herd immunity threshold you need to know the Rt, and that value is an estimate – a guess that can have a very big impact. Humanity has never been able to contain a disease that is transmitted through herd immunity between people. Measles and smallpox, for example, were only fought with vaccines. In the case of Covid-19, talk of herd immunity or early treatment only hampers efforts to fight the disease as social distancing measures no longer make sense to the population, ”he says.
Lotufo also claims that it is difficult to get accurate indicators from Manaus, making it difficult to precisely analyze the factors that favored the second wave of Covid-19 falls. In addition to the above hypotheses, such as re-infection or the emergence of a new, contagious variant, Lotufo mentions the possibility that some of the hospital stays may have come from people from the interior of the Amazon state in need of care due to the lack of beds in their communities in the capital seek – a phenomenon known as hospital admission invasion.
Projection versus reality
Among the experts who believed that Manaus had reached the herd immunity threshold by mid-2020 is Portuguese biomathematics Gabriela Gomes, currently professor at the University of Strathclyde (Scotland), who developed a model that takes into account the fact that individuals are a population have different degrees of susceptibility and exposure to the virus. The Gomes group includes researchers from the Institute for Biomedical Sciences (ICB) of the USP Marcelo Urbano Ferreira and Rodrigo Corder.
“If we compare the October hospital projections for Covid-19 in Manaus to the heterogeneous model with the homogeneous model used by researchers at Imperial College London [Reino Unido]We find that the current reality is halfway between the two projections. The curve is not as high as the British predicted and not as low as we estimate. I think that’s because all models have limitations, ”says Ferreira of Agência FAPESP.
Since this is a new disease, the dynamics of which are gradually being revealed, some important variables are still inaccurate in the calculations to be made.
“We worked with antibody data to estimate the number of cases. Regardless of the adjustment made, this is problematic. We now know that the amount of antibodies tends to decrease over time. In addition, infected people are still at risk of re-infection, which was initially unknown. Another limitation concerns the way in which we incorporate the parameters into the models that represent the relaxation of control measures. São Paulo, for example, switched from red to yellow between Natal and New Years. We also don’t know exactly to what extent people have resumed their normal lives or continue to adhere to distance guidelines. All of this makes modeling difficult, not to mention the emergence of new variants of the virus that are even more transmissible, ”says Ferreira.
The group is currently investigating how mobility restriction measures can affect the risk coefficient. This is because Gomes’ work is based on the premise that people are differently susceptible and exposed to the virus due to genetic and immunological factors and their lifestyle. “However, if everyone is locked up at home, that difference between individuals is reduced,” explains Ferreira.
The fourth and most recent phase of Epicovid’s epological survey, published in September 2020 and conducted in 133 Brazilian cities by researchers from the Federal University of Pelotas (UFPel), found that the epidemic is slowing in most of the country. According to the epidemiologist Pedro Hallal, coordinator of the initiative, the trend began to change in the final stage of the local elections. The impact size is measured in January when the fifth level of coverage of the Epicovid survey takes place.
“The two weeks before the second round were decisive. Then came the year-end parties and summer vacation. According to official statistics, the cases have been growing steadily at least until mid-December, but we have not yet returned to the levels when the moving average of deaths was over a thousand per day. It is difficult to say whether the growth trend will continue. The history of the pandemic is being built up on a daily basis. It all depends on how the population behaves, ”says Hallal.
The researcher complains about the lack of effective national policies to combat the disease. “We know what to do to make the curve drop quickly: extensive testing and contact tracing. This was never implemented in the country. Today, when a person is diagnosed, no one examines who they have been in contact with, and that should be mandatory. Other countries put a lockdown if cases increase and the virus quickly stops circulating and everything can be reopened. Here in Brazil we are doing a half-hearted isolation that is bad for public health and for the economy that is also half-hearted. “
For Hallal, the main policy to be planned in 2021 is vaccination, which he sees as the only way to achieve herd immunity without a mortality tragedy. Lotufo agrees and says he is optimistic.
“Vaccines from the Butantane Institute [desenvolvida pela empresa chinesa Sinovac] and the AstraZeneca laboratory [desenvolvida pela Universidade de Oxford, Reino Unido] They are good in efficiency, easy to use and made here in the country. If all goes well, we can vaccinate the population quickly, ”says Lotufo.
Croda believes there will not be enough doses to begin with to achieve herd immunity nationwide. For this, 80% of the population would have to be vaccinated.
“However, according to the World Health Organization, it is possible to significantly reduce hospital stays and deaths from vaccination with the highest risk of 20%. This will already be a big step forward, ”he says.