In a recently published article in Scientific reportsResearchers conducted an observational study among patients with coronavirus disease 2019 (COVID-19) and their close contacts enrolled in home health care services in Bangkok, Thailand. They evaluated T cell and neutralizing antibody responses six months after exposure to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
By late 2022, 75% of Thailand’s population had been vaccinated, mostly with viral vectors and technology-based COVID-19 vaccines. However, the Omicron SARS-CoV-2 variant infected 5% of the country’s population, but fortunately the fatality rate was lower than the fatality rate observed during the era of delta control (120 versus 300 per day).
The researchers predicted that people in Thailand would develop herd immunity after mass vaccination via vaccine or infection-induced hybrid immunity against SARS-CoV-2 infection. They also hypothesized that the low mortality rate during the 2022 Omicron outbreak in Thailand may have been due to herd immunity, hybrid immunity, and Omicron’s low virulence.
Moreover, close contacts of infected cases developed T-cell immunity against SARS-CoV-2, which kept Bangkok’s COVID-19-related death rates low, even among cases treated in home care centres. However, reluctance to receive the vaccine and the emergence of new variants of SARS-CoV-2 that evaded immunity impeded meeting the herd immunity threshold.
In this study, the researchers collected blood samples from all 79 participants from 15 randomly invited families from different urban areas in Bangkok. Of these, 34 had recovered from SARS-CoV-2 approximately four weeks before they were recruited into this study, while 45 had COVID-19 cases who had been in close contact with them.
At least one SARS-CoV-2 positive patient in each participating household must be a registered member of a home health care center in Bangkok between 1 and 31 August 2021, with at least one asymptomatic close contact living in the same care center. The researchers analyzed the T-cell response using an enzyme-linked immunosorbent spot assay (ELISpot).
During exposure to SARS-CoV-2, COVID-19 patients and their close contact(s) in each household were housed together in the same healthcare home, approximately 200 square metres.2. The study population consisted of 15 families with 11 individuals in each family, and 58% of them were female. More than 90% of the members were under the age of 60, and 81% had a body mass index (BMI) of less than 30. In addition, one-fifth of the study participants, 26.5% of patients and 15.5% of their close contacts, had the diseases. co-morbidities that have an increased risk of developing severe COVID-19.
The researchers observed a positive T-cell response to neuromyelitis optica (NMO). antigen In 11 cases out of 45 cases, or 24.4% of close contacts, indicating a previous infection. Remarkably, these 11 cases that showed T-cell responses were most likely asymptomatic.
Furthermore, the researchers observed a significant decrease in the levels of immunoglobulin G (IgG) receptor-binding domain (RBD) of unvaccinated patients six months after exposure to SARS-CoV-2, and that of close contacts was comparable. Interestingly, RBD IgG levels increased in a dose-dependent manner, but the T-cell response did not, a condition termed ‘T-cell exhaustion’.
On the contrary, the researchers observed a decrease in T-cell responses against the S antigen that increased with increasing doses of the vaccine, that is, with the third or fourth booster. Fortunately, studies have shown T-cell responses against the Omicron variant SARS-CoV-2 to be rapid after three months of boost. The type of vaccine may have confounded these responses.
Notably, 75% of the study participants had received an inactivated vaccine or a viral vector vaccine approximately three and a half months prior to their recruitment to this study. On the other hand, less than a third of the participants received a booster dose of the mRNA vaccine about a month before the study was recruited. Therefore, clinicians should closely monitor infections early after multiple vaccine dose boosters and manage them with special considerations, particularly in cases with a poor T-cell response.
Overall, the T-cell and antibody responses observed indicated hybrid immunity among vaccinated patients with prior SARS-CoV-2 infection and herd immunity among unvaccinated patients with prior COVID-19 infection. Likewise, closely vaccinated people with no history of COVID-19 have herd immunity.
Importantly, nine non-immune close contacts also showed a T-cell response against SARS-CoV-2(S) antigen. They may have developed herd immunity from an asymptomatic bout of infection. Most importantly, with a decrease neutralizing antibodies Against the Omicron variant, a T-cell-based vaccine that can generate diverse memory B cells against SARS-CoV-2 is needed.