On June 1, the World Health Organization approved the use of CoronaVac, China’s COVID-19 vaccine, to prevent infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
CoronaVac has been shown in clinical trials to prevent severe COVID-19 disease while showing a notable safety and immunogenicity profile. In countries like Chile that have approved CoronaVac, research has shown an increase in neutralizing antibodies 2 and 4 weeks after vaccination.
However, more data has surfaced on the decrease in immunity from COVID-19 vaccines six months after vaccination. To fight against this drop in protection, some countries have opted for booster injections to renovate the immune system.
In a recent study published on the preprint medRxiv* server, an international team of scientists tested CoronaVac as a booster injection and measured its effects on immunity. Their findings showed an increase in neutralizing antibodies and T cell response.
CoronaVac works as an inactivated virus vaccine. It is similar to traditional vaccines in that the dead virus is destroyed by the immune system where it learns to recognize it by creating antibodies specific to SARS-CoV-2. The CoronaVac vaccine is currently not approved by the United States Food & Drug Administration and the European Medicines Agency.
Neutralizing capacity of circulating antibodies after vaccination
About 129 participants who received two doses of CoronaVac from January to March 2021 were enrolled in the study. All participants also received a CoronaVac booster injection.
The highest point of neutralization occurred two weeks after the second dose of vaccine. However, the neutralizing capacity decreased 4 weeks after the second dose and continued to decrease 5 months after the full vaccination.
After participants received the booster, neutralizing antibodies increased and exceeded the highest peak observed for the first time after the second stroke.
Specifically, the booster resulted in a 12-fold increase in neutralizing antibodies over the neutralizing capacity observed 5 months after the second dose. In addition, the capacity to neutralize after the booster was 2 times greater than the capacity to neutralize 2 weeks after the second dose.
The peak neutralization capacity was four weeks after the booster. The peak of the booster dose was 18 times greater than what was observed 5 months after the second dose. In addition, the peak neutralizing capacity after a booster was 4 times greater than 2 weeks after the second dose.
The seropositivity after full vaccination was 100% in the participants, but after 5 months seropositivity dropped to 49.4% in the whole vaccinated group and to 35.7% in adults 60 years and older.
Seropositivity increased to its highest levels four weeks after the booster.
Four weeks later, seropositivity returned to 97.4% in the whole vaccinated group. Among the elderly, seropositivity increased to 95.2%.
Cellular responses after the CoronaVac booster
The research team observed a significant increase in CD4 cells+ Activation of T cells in participants of all ages who received the CoronaVac booster.
However, CD4+ T cell activation remained significantly elevated 5 months after the second dose of CoronaVac, indicating that the vaccination itself continued to promote CD4+ cellular responses over time.
CD4+ T cell levels were highest 4 weeks after the booster dose.
A significant rise in CD8+ GOAL+ T lymphocytes after the boost were also observed. There was no significant increase in IFN-γ, suggesting that CoronaVac stimulates CD8 reduction+ T cell responses.
“Our results suggest that a third dose of CoronaVac® supports activation of CD4 + T cells, which may confer protection or enhanced immune responses against the virus and prevent serious illness after exposure to SARS-CoV-. 2 “, the researchers concluded.
Limitations of the study
The study design as a whole had a reduced sample size when analyzing the ability to neutralize antibodies against SARS-CoV-2 with testing. As a result, samples with an undetermined concentration at the lowest dilution tested at 1: 4 were given a lower limit of quantification.
Information measuring the total antibody response of the SARS-CoV-2 spike protein and other SARS-CoV-2 proteins would provide more information on the humoral immune response after a COVID-19 booster injection.
medRxiv publishes preliminary scientific reports which are not peer reviewed and, therefore, should not be considered conclusive, guide clinical practice / health-related behavior, or treated as established information.