A new Japanese study has found that updated COVID-19 vaccines only work for people who were not previously infected with the virus. According to the researchers, vaccinated individuals who had a history of infection and received one of the updated shots actually experienced a slight decrease in protection against infection.
The study compared people who had received at least two doses of the original Pfizer and Moderna vaccines to those who received a bivalent shot in addition to the original doses. The researchers, from the Department of Health Care Administration and Management at Kyushu University and other institutions, stated that they did not find sufficient evidence of the effectiveness of bivalent vaccines among previously infected older adults.
According to Dr. Jeffrey Klausner, vaccinated individuals without prior infection showed an estimated 18.6 percent increase in protection against infection with a bivalent booster shot. However, the study also estimated that the added protection peaked at 26.5 percent and then dropped to 21.7 percent after 34 days. The study did not examine the long-term protection provided by the bivalent vaccines. The research was funded by the Japan Agency for Medical Research and Development and the Japan Society for the Promotion of Science.
The bivalent vaccines were introduced in the United States, Japan, and other countries to address the diminishing effectiveness of the original vaccines against emerging variants of the virus that causes COVID-19. However, these shots have since been replaced by new monovalent shots. In Japan, the new shots are available, but authorities are recommending them only to the elderly and people with underlying health problems. In the United States, officials recommend the new shots for virtually every person aged 6 months or older, regardless of whether they’ve been infected before. According to the U.S. Centers for Disease Control and Prevention, only 22 percent of adults and 11 percent of children have received one of the new shots.
Other studies have also examined the effectiveness of bivalent vaccines, with research from South Korea finding that one version of the bivalent shot was effective only in individuals without prior SARS-CoV-2 infection. European researchers estimated that bivalent vaccines initially provided strong protection against severe illness but that the protection dropped to near-zero over time.