Covid-19 vaccines (IMAGE) Johns Hopkins Medicine Caption A Johns Hopkins Medicine study finds booster doses of bivalent mRNA COVID-19 vaccines given every three to six months helps maintain a person’s ability to neutralize multiple SARS-CoV-2 strains, including XBB.1.5. Credit Public Domain Image Usage Restrictions No restrictions. License Public Domain Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.