Bivalent mRNA vaccines, containing elements from both the original wild type COVID strain and an updated component from the omicron strain, were designed to help improve vaccine-induced immunity against the omicron variant and subsequent subvariants.
These vaccines started replacing the older style monovalent boosters in the USA, Israel, and other countries since September, 2022.
“Our findings highlight the importance of new types of vaccines containing different variants of SARS-CoV-2, which are likely to induce broader immune responses and provide enhanced protection against severe outcomes,” explained study co-author Ronen Arbel, Community Medical Services Division, Clalit Health Services, Tel Aviv, Israel.
Israel has prioritised bivalent mRNA booster vaccines for people at high risk of severe COVID-19, primarily those aged 65 years or older, making it possible for the authors to perform a retrospective cohort study there to evaluate the effectiveness of the bivalent vaccine.
5,69,519 eligible participants were identified between September 27, 2022, and January 25, 2023. Of those, 1,34,215 (24 per cent) participants received a bivalent mRNA booster vaccination during the study period.
Hospitalisation due to COVID-19 occurred in 32 participants who received a bivalent mRNA booster vaccination and 541 who did not receive a bivalent booster vaccination. Analysis showed this to mean a 72 per cent reduction in hospitalisation risk in those receiving the bivalent booster. The adjusted Covid-related hospitalisation risk in the bivalent booster recipients was found to be down to 0.035 per cent from 0.124 per cent in the non-recipients. Therefore, the absolute reduction in risk was 0.089 per cent.
This meant that one hospitalisation due to COVID-19 was avoided for every 1118 people vaccinated.
The adjusted COVID-19 related death risk in the bivalent booster recipients was found to be down to 0.013 per cent from 0.040 per cent in the non-recipients. This meant an absolute reduction in death risk of 0.027 per cent or a 68 per cent relative reduction in death risk for the recipients.
That is, one death due to COVID-19 was avoided for every 3722 people vaccinated.
The study authors noted some limitations of their study, including the low numbers of hospitalisations and deaths.
They said that use of only the Pfizer bivalent vaccine means that generalisation of the results to other bivalent vaccines should be done with caution.
They also said that the study was not a direct comparison between bivalent and monovalent vaccines, since both were not administered at the same time in Israel during the study period.
“Bivalent mRNA booster vaccination in adults aged 65 years or older is an effective and essential tool to reduce their risk of hospitalisation and death due to COVID-19. Vaccination remains the primary tool for avoiding severe COVID-19,” said Arbel.
The study claimed to be one of the first ones to assess the Pfizer bivalent vaccine.