CoronaVac provides herd immunity in region of the Colombian Amazon

In a recent study published in the journal Tropical Diseases, Travel Medicine and Vaccines, researchers determine the effectiveness of the CoronaVac® vaccine in a Colombian Amazon population. To this end, the researchers observed robust protection against mortality and severe illness due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and that the indigenous population had achieved herd immunity through mass vaccination.

Study: Effectiveness of the CoronaVac® vaccine in a region of the Colombian Amazon, was herd immunity achieved? Image Credit: Seda Yalova / Shutterstock.com

Background

Herd immunity is defined as the resistance to the spread of an infectious disease within a given population. This phenomenon often occurs when a large part of the community becomes immune to that disease through previous infections or vaccinations. One of the critical advantages of reaching herd immunity is that it reduces the likelihood of infections among individuals with less or no immunity.

The coronavirus disease 2019 (COVID-19), caused by SARS-CoV-2 infection, continues to cause severe illness and death in many countries worldwide. As of January 23, 2022, over 60% of the world’s population has received at least one dose of a COVID-19 vaccine, which amounts to nearly 9.87 billion doses of COVID-19 vaccines that have been administered worldwide.

The disease burden of COVID-19 is expected to reduce as a result of protection provided from vaccine-induced immunity across populations, thereby bringing the world closer to possibly achieving herd immunity.

Despite these efforts, various developing countries have not been able to vaccinate as much of their populations. In Colombia, where the incidence of further infection spread and increased severity was high due to its proximity to Brazil, the Colombian Amazon was prioritized with the vaccination program. Of the vaccines administered in this country, including those offered by Sinovac, Pfizer-Biotech, AstraZeneca, Johnson & Johnson, and Moderna, 40% of the Colombian Amazon population have received the Sinovac CoronaVac®.

About the study

The present study aimed to evaluate the effectiveness of CoronaVac® in preventing severe COVID-19, hospitalization, and death in individuals residing within the Colombian Amazon who had been completely vaccinated against COVID-19.

COVID-19 severity is generally defined as mild, moderate, or severe disease. Whereas mild disease is associated with nonspecific symptoms such as fever, pain muscle, or general discomfort, moderate disease is often accompanied by clinical or radiological evidence of lower respiratory infection with compatible lung images. Severe COVID-19 typically is associated with a respiratory rate greater than 30/min, oxygen saturation less than 93%, pressure/inspired oxygen fraction (PaFi) values of less than 300, and infiltrates greater than 50%.

The current study was conducted between February 24, 2021, to August 10, 2021. A total of 7,856 participants were included in the current study, all of whom were older than 18 years old and resided in Mitú, the capital of Vaupés, Colombia, a region located in the southeast of Colombia (Amazonas) bordering Brazil.

Within Mitú, 60.4% of the inhabitants are predominantly indigenous. Of these, 99.9% (7,849 people) received two doses of the CoronaVac® vaccine.

The researchers presented the sociodemographic details of all study participants and details regarding those who were infected with SARS-CoV-2 infected post-vaccination and the level of severity of COVID-19 in the vaccinated population.

Study findings

Describing the incidence of SARS-CoV-2 infections after vaccination and the wave of the peak of cases, the researchers found that 5.7% of vaccinated individuals became ill, and only 0.1% of these require hospitalization. One death, which occurred in an individual over the age of 60, attributable to COVID-19, was reported among the vaccinated individuals.

The fatality rate before the vaccination was 2.2%. However, after vaccination, the fatality rate dropped to 0.22% in the immunized population.

Considering disease severity and vaccination effectiveness, the researchers observed 94.3% effectiveness against mild disease and 99.9% against severe infection. Furthermore, in the case of preventing death attributed to SARS-CoV-2 infection among the vaccinated group, 99.9% vaccine effectiveness was reported. The researchers noted that herd immunity might have been achieved through mass vaccination in this indigenous population based on this solid vaccine effectiveness.

Discussion

Moreover, the observations in this study correlate with previous studies in Brazil, though the data differed in the case of the older population. This difference in the protection could be because of the infection due to the SARS-CoV-2 P1 variant in Brazil. Likewise, compared to different study reports, the difference in the results and observed vaccine effectiveness can be explained due to different epidemiological scenarios.

Notably, the researchers also highlighted the longitudinal protection against morbidity and mortality rendered by the CoronaVac® vaccine, despite possible incidences of the Brazilian variant P.1 of SARS-CoV-2. Because genotypic surveillance is lacking in this population, the researchers could not conclusively account for the infections caused by the variant. Possible breakthrough infections from pediatric populations and responses to new variants have not been fully studied in this vaccinated population.

Conclusions

The current study demonstrated that herd immunity had been achieved through mass vaccination in this population, which has resulted in reduced severe infectious cases and mortality rate from COVID-19.

These findings support the need for continuing vaccination campaigns to provide better protection against infection and emphasize that herd immunity is achievable with maximum mass immunization. Encouragingly, data from this indigenous population suggest the morbidity and mortality rate was significantly reduced with CoronaVac®.