It's the Number of COVID Shots You Get, Not Combination, That Brings Immune Boost

THURSDAY, June 2, 2022 (HealthDay News) -- The number of COVID-19 vaccine doses received — not the combinations — is the key to strengthening your immune system to protect against different variants of the coronavirus, including Omicron, a new study suggests.

A team from the Chinese University of Hong Kong also determined that a third shot — a booster dose — is needed to prevent infection by the Omicron variant.

The effectiveness of individual COVID-19 vaccines has been proven, but the protection afforded by vaccine combinations is less clear, especially for at-risk groups.

Also, concerns about waning vaccine immunity and new variants makes it important to understand which vaccine combinations are most effective, the researchers explained.

For the study, the investigators analyzed 53 studies that included more than 100 million people. In all, they received 24 combinations of COVID-19 vaccine regimens using seven different mRNA and adenoviral vector vaccines.

The team found that three doses of any mRNA vaccine seem most effective (96%) against non-severe COVID-19 infections and most effective (95%) in reducing COVID-19-related hospital admission. Pfizer-BioNTech and Moderna are mRNA vaccines.

Using an mRNA booster after two doses of adenovirus vector vaccines was 88% effective. Johnson & Johnson's COVID shot is a vector vaccine.

Any three-dose regimen of the same vaccine type or a combination of different vaccine types led to greater immunity in all age groups than two doses of the same vaccine, Peter Pak-Hang Cheung and colleagues concluded. Cheung is with the chemical pathology department at Li Ka Shing Institute of Health Sciences.

A third mRNA booster dose as part of a regimen of the same or different vaccine types provided much better protection for immunocompromised patients than two doses, according to the study.

The findings were published May 31 in the BMJ.

It's not clear whether three vaccine doses protect against death from COVID-19, the study authors noted in a journal news release.

Due to limited information in the studies, the researchers weren't able to assess the ideal time interval for a prime boost or boosting regimens.

But they said the study does show that three doses of the same or different types of vaccines work comparably well in preventing infections, even against different variants, and the findings should help guide public health decisions about COVID-19 vaccination.

More information

For more on COVID-19 vaccines, go to the U.S. Centers for Disease Control and Prevention.

SOURCE: BMJ, news release, May 31, 2022