Durability of bivalent boosters against omicron subvariants

Danyu Lin, Yangjianchen Xu, Yu Gu, Donglin Zeng, Shadia Khan Sunny, Zack Moore

Abstract:

On September 1, 2022, the Moderna and Pfizer-BioNTech bivalent Covid-19 vaccines replaced their monovalent counterparts as booster doses for individuals 12 years of age or older in the United States. We previously reported surveillance data from the state of North Carolina on the effectiveness of these two bivalent boosters over the first 3 months of deployment. Here, we present two additional months of data to show the durability of protection conferred by these two bivalent boosters when the omicron’s BQ.1/BQ.1.1 and XBB/XBB.1.5 strains became predominant.

We used data on booster doses and clinical outcomes from September 1, 2022 to February 10, 2023 for all North Carolina residents 12 years of age or older. During this time period, 6,306,311 participants were eligible for bivalent boosters, and 1,279,802 received them; 253 of the 2,208 Covid-19 related hospitalizations and 79 of the 867 Covid-19 related deaths occurred after receipt of the bivalent booster.

We considered two different but non-exclusive outcome measures: severe infection resulting in hospitalization and severe infection resulting in hospitalization or death. We fit the Cox regression model with a timevarying hazard ratio of severe infection for each additional booster dose received, adjusting for the baseline characteristics shown in Table S1. We defined the booster effectiveness as one minus the hazard ratio, multiplied by 100%.

The estimation results are shown in Figure 1. Effectiveness against severe infection resulting in hospitalization reached a level of 62.3% (95% CI, 36.9 to 77.5) after 2 weeks and decreased to 47.4% (95% CI, 31.4 to 59.7), 43.4% (95% CI, 34.1 to 51.4), and 36.0% (95% CI, 9.0 to 55.0) after 4 weeks, 10 weeks, and 20 weeks, respectively.

Effectiveness against severe infection resulting in hospitalization or death reached a level of 67.4% (95% CI, 46.2 to 80.2) after 2 weeks and decreased to 47.5% (95% CI, 32.6 to 59.2), 44.3% (95% CI, 35.7 to 51.7), and 38.4% (95% CI, 13.4 to 56.1) after 4 weeks, 10 weeks, and 20 weeks, respectively.

We also analyzed the data separately for participants who received bivalent boosters before November 1, 2022 (when BA.4/BA.5 were predominant) and after November 1,2022 (when BQ.1/BQ.1.1 became more prevalent and then were gradually replaced by XBB/XBB.1.5). The results are also shown in Figure 1. The ramping and waning patterns were broadly similar between the two booster cohorts.

In summary, bivalent boosters provided durable additional protection against hospitalization and death from omicron infection in persons who had previously been vaccinated or boosted. Although the two bivalent vaccines target the BA.4/BA.5 strains, they were also effective against the BQ.1/BQ.1.1 and XBB/XBB.1.5 strains.