A new methodology can be applied to track infectious diseases and calculate herd immunity
Research published in the scientific journal eLife concluded that it is possible to calculate the proportion of the population previously infected (seroprevalence) with SARS-CoV-2 from blood bank donor samples. With the results, in addition to compiling a kind of “snapshot” of the Covid-19 epidemic in Brazil during the first year, the researchers indicated that the new methodology could be applied to track other types of infectious diseases and calculate herd immunity.
Currently, seroprevalence calculations are made using random population samples, a method considered more expensive and more difficult to perform periodically and in real time. This type of surveillance is important to understand the characteristics of the epidemic and to establish public policies, for example, revealing where prevention and treatment measures are operating.
Scientists from the United Kingdom-Brazil Joint Center for Arbovir Virus Discovery, Diagnosis, Genomics and Epidemiology (CADDE) analyzed 97,950 donor blood samples for immunoglobulin G (IgG) antibodies from the eight most populous capitals: Belo Horizonte, Curitiba, Fortaleza and Manaus, Recife. , Rio de Janeiro, El Salvador and Sao Paulo. The period analyzed was from March 2020 to the same month of 2021.
The results showed that the Covid-19 epidemic was heterogeneous in Brazil, infecting different populations at different times. In general, the disease first began to affect men and young adults.
Initially, some lines of research believed that everyone was injured at the same time, but we have shown that this is not true. Regarding the description of the epidemic, we conclude that there is a high variation in Brazil, with differences in infection by groups and a wide variation in mortality by case. This was a result that we did not expect, researcher Carlos Augusto Preti Jr., from the Polytechnic University of São Paulo (Poli-USP), author of the corresponding article, told Agência Fapesp.
The study was part of the Prete Junior Doctorate, whose supervisor is Professor Vitor Heloiz Nascimento, from Poli, and as co-director Ester Sabino, professor at the Faculty of Medicine of the University of the South Pacific and head of CADDE in Brazil.
Both are also authors of the work, which was supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (Fapesp) through CADDE and the Prete Junior Scholarship, in addition to receiving funding from the Instituto Todos pela Saúde. Scientists from Imperial College London (UK) and the University of Oxford also participated.
Recently, in another article published in the Journal of Vaccines, the group has already shown, from the analysis of blood bank samples, that it is possible to predict the transmission of the delta variant (discovered in India in 2020 and originally named B.1.617 .2) in Brazil. . In this case, they measured the amount of IgG antibodies in the population by performing anti-S microparticle assays, that is, to detect antibodies capable of binding to the spike protein of the coronavirus. With this, they were able to correlate the protection of the vaccine with the incidence of the delta variable and the level of mortality.
Covid In Brazil
The Covid-19 epidemic in Brazil was one of the largest in the world: in early October 2022, more than 34.7 million cases and 687,000 deaths from the disease were recorded. However, these numbers mask differences in the epidemic between regions and subpopulations, as well as underreporting of the proportion of the population previously infected with the virus. Estimating this relationship is important to predict the impact of future waves of the epidemic caused by new variables.
In the survey, scientists estimated seroprevalence over time for the eight capitals using samples from blood donors to obtain estimates disaggregated by age and sex. They also made estimates of infection death rates by age – calculating the so-called IFR (infection death rate), defined as the number of deaths from infection – and hospital infection rates in these cities.
Each of the eight blood banks had a monthly quota of 1,000 samples analyzed. In order to be representative, they were chosen so that the spatial distribution of the donors’ place of residence corresponds to the division of the population by administrative regions in each municipality.
Since the Brazilian norm states that blood donation samples must be kept for six months, the researchers were able to select and test frozen samples between February (before the epidemic began in the country) and July 2020. After that, it began your selection and testing. Tested in real time.
Tests were performed that detect IgG antibodies against the nucleocapsid, a protein found in SARS-CoV-2. However, this type of test may show a decrease in antibody detection sensitivity over time. That is, as the epidemic progressed, the crude proportion of people who tested positive decreased, which lowered the actual infection rate.
To correct for this decrease in antibodies, a Bayesian seroconversion model was developed based on data from frequent donors, that is, from individuals who donate blood several times a year, as well as from a group of SARS-CoV-positive convalescent plasma donors. 2 asymptomatic not admitted to hospital. “This was important because some studies at the beginning of the pandemic suggested correction only by plasma donors. However, we have shown that frequent donors are more representative of the population of each municipality”, explained Prete Junior.
The researcher affirms that this model was not applied in the work that the group published in the journal Science, in December 2020, with information from the Manaus blood bank because there was not enough data at that time. In this study, coordinated by Ester, it was estimated that 76% of the manauaras did have immunity to the new coronavirus, but before the entry of other variants of SARS-CoV-2, such as gamma (P.1), later considered more aggressive and deadly.
“Shortly after we published the article, there was a second wave of Covid-19 in Brazil. At the time, many believed that there was no further infection. We now confirm that it is possible to use blood donor samples to calculate seroprevalence to monitor other diseases, provided that Adjustments, such as seroconversion correction and attack rate estimation for each age group and sex using spatial sampling.
The results showed that the SARS-CoV-2 attack rate in December 2020, before gamma control, ranged from 19.3% in Curitiba to 75% in Manaus. The seroprevalence rate was consistently lower in women and donors older than 55 years.
Cities with a higher seroprevalence also had a higher mortality rate (the number of deaths per capita). Between March 1, 2020, and March 31, 2021, the age-standardized mortality rate ranged from 1.7 deaths per 1,000 population in Belo Horizonte to 5.3 deaths per 1,000 in Manaus, which had double the mortality rate in Fortaleza, the city with the second highest rate Mortality rate among those analyzed.
The case fatality rate (IFR) also differed between cities, ranging from 0.24% in Manaus to 0.54% in Curitiba, and the infection-related mortality rate increased steadily. To avoid the problem of underreporting of deaths from Covid-19, the IFR was estimated using the total number of deaths from severe acute respiratory infection, excluding deaths from SARS of other aetiologias.
COVID-19 In Manaus
Gamma appeared in November 2020 and its prevalence among the remaining variants grew rapidly to 87% on January 4, 2021, with a high incidence of infection again. Therefore, IFR and attack rate were estimated separately for the second wave of SARS-CoV-2 in Manaus, when gamma was dominant.
The study showed that the proportion of the population infected in the second wave of Manaus reached 37.5%, compared to 75% in the first wave. The rate of hospitalizations for infection grew during the second wave in the capital of Amazonas, indicating an increase in the severity of disease caused by the variant compared to previous waves.
The largest outbreak of Covid-19 linked to the collapse of the health system in Manaus meant that the death rate from infection in the range was at least 2.91 times higher than in the first wave.