ABSTRACT
Background: A nationwide vaccination program against coronavirus disease 2019 (COVID-19) was started in Mongolia 4 months after the first local transmission, which occurred in November 2020. Previous studies have reported that two doses of COVID-19 vaccine result in increased antibody against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A study was conducted in Mongolia 2 weeks after the second vaccine dose. In the present study, the serum levels of antibodies of individuals 6 months after natural SARS-CoV-2 infection were compared with those of individuals who had not been infected or had been infected but had received two doses of vaccine, including BNT162b2, ChAdOx1 n-CoV-19, Gam-COVID-Vac, and BBIBP-CorV, which were used for COVID-19 in Mongolia. Methods: Of the 450 participants in this study, 237 (52.66%) were female and 213 (47.33%) were male. Four hundred people with or without SARS-CoV-2 infection who received two doses of 4 different COVID-19 vaccine participated in the vaccine groups and vaccine plus SARS-CoV-2 infection groups (50 in each group) and 50 individuals previously infected with SARS-CoV-2 participated in the unvaccinated group. Total antibody against SARS-CoV-2 infection, anti-SARS-CoV-2 N and S protein human IgG, and antibody inhibiting RBD-ACE2 binding were tested. Results: In the BNT162b2 vaccine group, total antibody against SARS-CoV-2 remained constant until 6 months, while the other vaccine groups showed a significant decrease, as compared to the unvaccinated group. The level of anti-SARS-CoV-2 S-RBD protein IgG was significantly increased in the ChAdOx1 n-CoV-19, Gam-COVID-Vac, and BNT162b2 vaccines groups as compared to the unvaccinated group. Participants in the BNT162b2 vaccine group had higher ACE2 inhibition efficiency compared to the other vaccine groups as well as the unvaccinated group. Conclusions: The BNT162b2 vaccine showed the highest level of antibody against SARS-CoV-2, followed by the BBIBP-CorV, Gam-COVID-Vac, and ChAdOx1 n-CoV-19 vaccines. The level of antibodies was increased in people infected with SARS-CoV-2 after vaccination, as compared to uninfected but vaccinated individuals.
ABSTRACT
The 2019 novel coronavirus disease (COVID-19) is the disease that has been identified as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), but the prophylactic treatment of SARS-CoV-2 is still under investigation. The effective delivery of eukaryotic expression plasmids to the immune system's inductive cells constitutes an essential requirement for generating effective DNA vaccines. Here, we have explored the use of Salmonella typhimurium as vehicles to deliver expression plasmids orally. The attenuated Salmonella phoP was constructed by the one-step gene inactivation method, and plasmid-encoded the spike protein of SARS-CoV-2 was transform into the Salmonella phoP by electroporation. Western blot experiment was used for the detection of SARS-CoV-2 expression on 293T cells. Wistar rats were immunized orally with Salmonella that carried a eukaryotic expression plasmid once a week for three consecutive weeks. The ELISA was performed to measure the SARS-CoV-2 specific IgG at rat's serum samples. pSARS-CoV-2 can be successfully expression on 293T cells, and all immunized animals generated immunity against the SARS-CoV-2 spike protein, indicating that a Salmonella-based vaccine carrying the Spike gene can elicit SARS-CoV-2-specific secondary immune responses in rats. Oral delivery of SARS-CoV-2 DNA vaccines using attenuated Salmonella typhimurium may help develop a protective vaccine against SARS-CoV-2 infection.