In a randomized, placebo-controlled trial of a COVID-19 booster vaccine in transplant recipients, scientists at University Health Network’s (UHN) Ajmera Transplant Centre have reported that a third dose of the Moderna vaccine is safe and induces a robust immune response.
“We knew from previous studies that two doses of existing vaccines are not enough to produce a strong immune response against COVID-19 in transplant patients,” says Dr. Deepali Kumar, Director of Transplant Infectious Diseases at UHN and joint-senior author of the study published in the New England Journal of Medicine. “Based on our study, a third dose of the Moderna vaccine appears to increase protection in transplant recipients.”
The clinical trial was randomized, double-blind and placebo-controlled—providing a level of rigor that is considered the gold standard in medicine. The trial enrolled 120 transplant patients that had not previously contracted COVID-19 and had received two doses of the Moderna vaccine. At the two-month mark after their second dose, half of the participants received a third shot of the Moderna vaccine and the other half received a placebo.
The researchers measured whether the booster shot increased antibody levels against the spike protein of the SARS-CoV-2 virus to greater than 100 U/ml—a level that is believed to provide at least 50% protection against developing COVID-19 based on previous studies. After three doses, only 18% of the participants in the placebo group showed this response, whereas 55% of those in the vaccine group met or exceeded these antibody levels.
The research team also looked at whether the booster shot led to the production of neutralizing antibodies—antibodies that can bind to and neutralize the virus. In the Moderna group, 60% tested positive for at least some ability to neutralize the virus, compared to only 25% of participants in the placebo group.
The researchers also found higher levels of T-cells that can specifically target the virus in the Moderna group compared to the placebo group. T-cells are a component of the immune system that can help prevent severe disease by either killing infected cells or promoting responses by other parts of the immune system.