Antibody avidity is an important parameter to evaluate immune response, being useful to evaluate vaccine responses and helping to distinguish acute and latent infection. The antibody avidity can be measured by different methods, yet the most common is a modified ELISA. The utilization of commercial kits or in-house methods to evaluate antibody avidity have been adopted more and more, although the lack of standardization between different assays may generate a lot of variation in the process, making it hard to compare the results generated.

Rhipicephalus microplus is a vector of cattle tick fever, a disease caused by the protozoans Babesia bovisand B. bigemina, and also anaplasmosis, produced by the Rickettsiales Anaplasma marginale. These tick-borne pathogens cause considerable losses to Brazilian livestock breeders and represent an obstacle to the expanded use of taurine breeds due to their higher sensitivity to ticks and hemoparasites compared to zebu breeds. Differences in the susceptibility to hemoparasites were also verified within breeds, suggesting that may be possible to select a most resistant phenotype. Therefore, repeatability of R. microplus counts and copy number of hemoparasites DNA were estimated, along with correlations between themselves, aiming to verify if those measures can be used as parameters to classify animals according to their parasite resistance degrees. Forty-two Canchim females kept on pastures naturally infested by ticks were evaluated for the level of infestation by R. microplus and infection by B. bovis, B. bigemina, and A. marginale. Twenty-four evaluations were performed once a month, for adult female ticks counts and blood samplings. The experimental period was divided into four phases, according to the animals age range: Phase 1: 8 to 13 months (collections 1 to 6); phase 2: 14 to 19 months (collections 7 to 12); phase 3: 20 to 25 months (collections 13 to 18), and phase 4: 26 to 31 months (collections 19 to 24). Blood samples were submitted to absolute quantification of hemoparasites DNA sequences using qPCR. The hemoparasite and tick counts data were transformed for normalization and were analyzed using mixed models. Among three species of hemoparasites studied, A. marginale presented the highest level of infection. During phase 3, B. bigemina presented higher infection levels (p < 0.05) compared to B. bovis, whereas no differences were observed in other phases. Estimated repeatabilities for parasite infection levels varied from low to moderate during our experiment. There were low correlations between tick counts and parasite infection levels, and between parasite infection levels from different species by themselves. Based on these results, under conditions of the present study, we suggest that it is possible to identify animals presenting a most resistant phenotype against infection by both hemoparasites and ticks. Moreover, the animal age may be an important factor related to resistance against these pathogens. The data obtained shed more light on the resistance to hemoparasites studied.

The Enzyme-Linked Immunosorbent Assay is a versatile technique, which can be used for several applications. It has enormously contributed to the study of infectious diseases. This review highlights how this methodology supported the science conducted in COVID-19 pandemics, allowing scientists to better understand the immune response against SARS-CoV-2. ELISA can be modified to assess the functionality of antibodies, as avidity and neutralization, respectively by the standardization of avidity-ELISA and surrogate-neutralization methods. Cellular immunity can also be studied using this assay. Products secreted by cells, like proteins and cytokines, can be studied by ELISA or its derivative Enzyme-linked immunospot (ELISpot) assay. ELISA and ELISA-based methods aided the area of immunology against infectious diseases and is still relevant, for example, as a promising approach to study the differences between natural and vaccine-induced immune responses against SARS-CoV-2.

Humoral response to SARS-CoV-2 has been studied, predominantly the classical IgG and its subclasses. Although IgE antibodies are typically specific to allergens or parasites, a few reports describe their production in response to SARS-CoV-2 and other viruses. Here, we investigated IgE specific to receptor binding domain (RBD) of SARS-CoV-2 in a Brazilian cohort following natural infection and vaccination. Samples from 59 volunteers were assessed after infection (COVID-19), primary immunization with vectored (ChAdOx1) or inactivated (CoronaVac) vaccines, and booster immunization with mRNA (BNT162b2) vaccine. Natural COVID-19 induced IgE, but vaccination increased its levels. Subjects vaccinated with two doses of ChAdOx1 exhibited a more robust response than those immunized with two doses of CoronaVac; however, after boosting with BNT162b2, all groups presented similar IgE levels. IgE showed intermediate-to-high avidity, especially after the booster vaccine. We also found IgG4 antibodies, mainly after the booster, and they moderately correlated with IgE. ELISA results were confirmed by control assays, using IgG depletion by protein G and lack of reactivity with heterologous antigen. In our cohort, no clinical data could be associated with the IgE response. We advocate for further research on IgE and its role in viral immunity, extending beyond allergies and parasitic infections.

Since late 2019 and early 2020, with the emergence of the COVID-19 pandemic, scientists are rushing to develop treatment and prevention methods to combat SARS-CoV-2. Among these are vaccines. In view of this, the use of animals as experimental models, both to investigate the immunopathology of the disease and to evaluate the efficacy and safety of vaccines, is mandatory. This work aims to describe, through recent scientific articles found in reliable databases, the animal models used for the in vivo testing of COVID-19 vaccines, demonstrating some possibilities of more advantageous/gold-standard models for SARS-CoV-2 vaccines. The majority of the studies use rodents and primates. Meanwhile, the most adequate model to be used as the gold standard for in vivo tests of COVID-19 vaccines is not yet conclusive. Promising options are being discussed as new tests are being carried out and new SARS-CoV-2 variants are emerging.