Herd Immunity in Brazil-Amazon-COVID-19
When a certain % of the population becomes immune either due to contraction of the disease or by vaccination, a threshold is reached, and the disease cannot infect others, since the available section would have either antibodies produced naturally or artificial means. For existing virus families, we know the threshold % and the means to achieve, and since COVID-19 is a very new virus, it is a fast-developing one. Manaus which is located in the Brazilian Amazon had an unusually high infection rate, which limited further infection among the population, and now the virus-positive caseload has dropped.
Simple epidemiological models state the threshold at 60% of the population becoming immune for herd immunity. Since we have a heterogeneous population, where if you have a younger age group and kids who are naturally immune to this virus, and at-risk older population, it varies from 20–43%. Europe and North America have <20% of cumulative infection, and widespread adoption of non-pharma measures to control the virus also has to be taken into account. The virus would stop spreading if it has fewer people to infect or by measures like social distancing, usage of masks, etc. The slow spread of the virus cannot be attributed to herd immunity if it is the latter slowing the infection rate.
Brazil has one of the worst global epidemic rates in the world, and the Manaus region has been the worst hit. After the epidemic peak in May, the number of cases has decreased despite the relaxation of social norms. Seroprevalence estimates in blood donors in Brazil are being used to predict herd immunity achievement in the region. We need to infer the true prevalence of infection from antibodies via an antibody test. For an asymptomatic patient, there are weaker antibodies generated and hence would not be recorded in serological studies.
In COVID-19 hospitalized patients, tests were done a month after symptoms, and the sensitivity rate was over 90%. This also implies that over 10% of hospitalized COVID patients do not develop detectable antibodies. Sensitivity was at 85% for mild symptomatic patients. With more days, antibodies prevalence ebbs, and this is one confusion among vaccine suppliers, of how long the immunity will last in the body against the virus. If natural antibodies ebb after 130+ days, it should be soon how long vaccine-induced antibodies will help.
This gives a corollary that, you should get a severe virus for your antibodies to develop but not severe enough to burn you down.
How do you estimate the immunity levels of a population?
We cannot take a blood sample from every section of the population, it is both time-consuming and costly, hence this study used blood samples from blood donors at a blood bank in the Brazil-Amazon region. This is voluntary and represents a cross-section of the population, also there is no additional cost/time involved. Antibodies count was low, < 1%in the February-March-2020 time frame since the first cases were only detected in March. It was 5% in April, 44% in May, and 51% in June. This was in line with cumulative deaths due to the virus.
It is said between 44–66% of the Manaus population was infected with COVID-19. Social distancing was at the same level as of Sao Paulo, as confirmed from cell phone transmission data, but the spread was quick in Manaus vs other cities in Brazil. Based on the detailed study, it is assumed that the herd immunity threshold is 60% in Manaus. The waning of antibodies following the peak of the epidemic is a cause for concern. Any second wave or re-introduction of infection among the population needs to be monitored.
Another aspect is if only healthy blood donors between the age group of 18–69 can be blood donors, and individuals without recent infection are sampled in the survey, these results might not be representative of the total population.
A household survey was also conducted which corroborated the results from the blood donor survey. Sampling weights were done based on the zip codes from the city, such that entire city region blood samples are validated to give the survey more authenticity.
With the talks of herd immunity only existing in theory, this study aims to change that it is possible and does exist in reality in a small region in Brazil. There are caveats that, if there is a second wave, and the duration of antibodies generated or any other complications need to be studied in the future.
Reference: https://www.medrxiv.org/content/10.1101/2020.09.16.20194787v1.full.pdf
