The rapid spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) caused the coronavirus disease 2019 (COVID-19) pandemic. Since the start of the pandemic, several variants of SARS-CoV-2 have emerged with different rates of transmission, virulence, and ability to evade immune responses elicited either by COVID-19 vaccination or natural infection compared to to the ancestral variant.
Study: Hybrid immunity at 6 months after exposure to SARS-CoV-2 in people in the community treatment program. Image Credit: Cinefootage Visuals / Shutterstock.com
The SARS-CoV-2 Delta variant was the dominant strain circulating in Thailand in mid-August 2021. The spread of this variant has dramatically increased the number of COVID-19 cases and deaths.
During this period, the number of daily cases reached around 20,000, with more than 300 deaths reported each day. The subsequent decline in COVID-19 cases occurred following the implementation of pharmaceutical and non-pharmaceutical measures.
Previous studies have indicated that vaccination against COVID-19 plays an important role in reducing daily infections and hospitalizations due to severe infection. According to the United States Centers of Disease Control and Prevention (CDC), vaccination against COVID-19, especially when messenger ribonucleic acid (mRNA) vaccines are used, has significantly reduced the death rate of the COVID-19.
Most of the Thai population was vaccinated with inactivated vaccines, with a small percentage given an mRNA vaccine as a booster dose.
The emergence of the SARS-CoV-2 Omicron variant in Thailand caused an increase in daily infections which reached nearly 50,000 per day. However, the number of deaths due to SARS-CoV-2 infection remained low, at around 120 per day.
Scientists have previously speculated that herd immunity will develop after a majority of the population has been vaccinated or recovered from COVID-19. However, the herd immunity threshold could not be reached due to the emergence of new variants, as well as reluctance to vaccinate.
A study from the UK coined the concept of “hybrid immunity”, which refers to the protection conferred by vaccination against COVID-19 and natural infection. Previous studies have indicated that hybrid immunity can protect individuals against symptomatic infection.
About the study
In a new study being reviewed in the journal Scientific Reports and currently available on the Research Square* preprint server, the scientists hypothesize that lower death rates in COVID-19 patients may be due to hybrid immunity, as well as reduced severity of Omicron infections.
The current study included 79 participants from 15 families registered in a Bangkok Home Care Service database between August 1, 2021 and August 31, 2021. In this study cohort, 34 people had recovered from COVID -19 at least four weeks before registration. , while the remaining 45 participants were in close contact with COVID-19 patients.
T-cell responses against neuromyelitis optica (NMO) antigens were detected by the interferon release assay in 11 of 45 close contacts six months after exposure to SARS-CoV-2. The rate of asymptomatic COVID-19 was estimated at 24.4%.
SARS-CoV-2 receptor binding domain (RBD) immunoglobulin G (IgG) antibody levels, as well as T cell responses against the SARS-CoV-2 spike protein, after second dose of COVID-19 vaccine, were associated with comparable rates of immunity between COVID-19 patients and close contacts. Thus, participants who received a COVID-19 booster vaccination without any prior history of COVID-19 also received hybrid immunity.
The antibody response to RBD IgG (Figure 2A) demonstrated the correlation between antibody levels and neutralization capacity to the alpha variant of the SAR-CoV-2 virus (R = 0.5571, P
A similar level of immunity was seen in people who were in close contact with asymptomatic and symptomatic infected people.
Previously, long-term observational data were not available regarding the rate of reinfection in people exposed to SARS-CoV-2. In the present study, none of the participants were reinfected with SARS-CoV-2 at enrollment. Picture 3.
Immune response against SARS-CoV-2 viral antigens compared between close contacts with and without asymptomatic infection, antibody response (RBD IgG) P=0.1922 t=1.325, df=43 (Figure 3A), T cell response against the spike protein P=0.5325, t=0.6293, df=43 (Figure 3B). RBD IgG = SARS-CoV-2 receptor binding domain immunoglobulin G, AU/ml = arbitrary units per milliliter.
Most of the study participants had received a heterogeneous booster vaccination against COVID-19 that included viral vector vaccines, inactivated vaccines and mRNA vaccines. To this end, reduced T cell responses against the spike protein and higher levels of IgG RBD were observed after the booster vaccination.
After a short period of exhaustion, T cells can recover three months after receiving a booster dose of COVID-19. This observation is consistent with a previous study that reported an effective retained immune response after heterogeneous immunization against the Omicron variant.
Previous studies have revealed that individuals may possess varying levels of cellular and humoral immune responses during similar viral infections. Since the majority of study participants received an mRNA vaccine as a booster dose, this could impair T-cell function after vaccination.
Thus, the authors strongly recommend multiple booster doses of the COVID-19 vaccine only for immunocompromised individuals with poor T cell responses. T cell-based vaccines would also provide increased therapeutic benefit, especially for these patients.
The current study found that antibody and cellular responses define immunity to COVID-19 in society. Taken together, these responses may confer long-term hybrid immunity following booster vaccination against symptomatic and asymptomatic COVID-19.
An important limitation of the present study is its small size, which limits the generalizability of its results. In addition, the immune response was disrupted by vaccination.
Research Square publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice/health-related behaviors, or treated as established information.