Despite his own highly contagious variant, is Manaus on the verge of achieving collective immunity?

The herd immunity threshold is the proportion of a population that must be immunized against an infectious diseaseeither by natural infection or by vaccination, so that in the absence of additional preventive measures, new cases decrease and the number of effective reproduction falls below unity.

This fundamental epidemiological parameter is still unknown for COVID-19, of recent appearance, and mathematical models have predicted widely divergent results. Population studies using antibody tests to infer total cumulative infections can provide empirical evidence for the level of population immunity in severely affected areas.

In a new study of the University of San Pablo work has been carried out to analyze how the transmission of SARS-CoV-2 in Manaus, located in the Brazilian Amazon, it increased rapidly in March and April and decreased more slowly from May to September. In June, one month after the epidemic peak, 44% of the population was seropositive for SARS-CoV-2, which equates to a cumulative incidence of 52%, after correcting for the false negative rate antibody test.

There is no consensus on the proportion of a population that should be infected with SARS-CoV-2 before herd immunity is achieved. Estimates of this threshold can help shed light on certain aspects of public health policy, including decisions to reopen the company and the deployment and impact of vaccination campaigns. Given a baseline reproduction index (R 0) of 2.5, the theoretical herd immunity threshold for SARS-CoV-2 in simple epidemiological models is approximately 60%. However, models that take into account the heterogeneous mixture of populations predict lower values, ranging from 20% to 43%. The herd’s threshold of immunity, along with social distancing and other control measures, determine the ultimate extent of the outbreak.

Antibody prevalence studies use serological tests to measure the proportion of a population with evidence of previous infection. When conducted in a particular setting, a serial cross-sectional seroprevalence study can provide empirical evidence for the ultimate size of the epidemic. Although many antibody prevalence studies have been conducted in Europe and North America, comparatively low estimates of the infections that have accumulated there (typically <20%) cannot be considered to reflect the immunity of the herd due to the widespread adoption of effective non-pharmaceutical control measures. In these places.

On the other hand, Brazil has one of the fastest growing COVID-19 epidemics in the world, being the Amazon, in the north of Brazil, the most affected region. Manaus is the capital of the state of Amazonas with a population of over two million and a population density of 158 inhabitants / km.

The first case in Manaus was confirmed on March 13 last year and was followed by an explosive outbreak. The excess mortality in Manaus in the first week of May was 4.5 times higher than the previous year. The peak of the pandemic in early May was followed by a sustained decline in cases and deaths despite the easing of control measures.

Although the ideal design for determining the prevalence of infection is a population-based sample, this approach is time consuming and expensive. Routine blood donation can be a logistically manageable alternative. For the study of the University of San Pablo The specialists presented monthly cross-sectional estimates of seroprevalence among blood donors in Manaus that cover the first seven months of transmission in Brazil. and they correlate these results with the full epidemic curve in the Amazon region. They compared these estimates with parallel findings from São Paulo (southeastern Brazil), where the first cases of COVID-19 were detected in Brazil.

Don’t drown in the flood

“Our results show that between 44% and 66% of the population of Manaus were infected with SARS-CoV-2 during the epidemic -explains Lewis F Autobuses, lead author of the study. The lower estimate does not take into account false negatives or decreased antibodies; the higher estimate explains both. The high death rate and the rapid and sustained decline in cases suggest that the immunity of the population has played an important role in determining the extent of the outbreak in the city.“.

The non-pharmaceutical interventions implemented in the city were similar to those in other cities in Brazil, including São Paulo. They were implemented at the end of March before the epidemic took off. Additionally, cell phone mobility data showed a marked increase in physical distance from mid-March. Therefore, it is not known what explains the rapid transmission of SARS-CoV-2 in Manaus.

Possible explanations include unfavorable socio-economic conditions, overcrowding of dwellings, limited access to drinking water. and reliance on high-risk boat trips, in which overcrowding leads to accelerated contagion, similar to that seen on cruise ships. The young population with potentially weak pre-existing immunity to SARS-CoV-2, as well as the circulation of several viral lineages introduced from several locations may have contributed to the large scale of the epidemic.

“Our results cannot be extrapolated directly to other contexts due to differences in population demographics, behavior, vulnerability to infection, as well as implementation and compliance with non-pharmaceutical measures”, warned the specialist. The population proportion works in conjunction with these factors to skew the actual number of spawns below unity. In fact, the herd immunity threshold in Manaus would be 60% and the final size of the epidemic 89%. This assumes that the population is homogeneous and is not subject to effective INP. Homogeneous mixing is unlikely to be a valid assumption; Yes heterogeneous exposure or susceptibility to infection may explain why the estimated final size of 44-66% infected is less than 89% ”.

Researchers observed a drop in antibodies after the epidemic peak in Manaus. These results have important implications for the design and interpretation of antibody prevalence studies. In order to estimate the total cumulative infections in a population, the test chosen should ideally detect a durable component of the humoral response to SARS-CoV-2.

Manaus can act as a sentinel to determine the longevity of the population’s immunity and the frequency of re-infections. An additional antibody surveillance strategy would be to follow local versus imported cases, with a relative increase in local cases, suggesting that the immunity of the population no longer prevents transmission in the future.

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