In a recent study posted to the medRxiv* preprint server, researchers explored the alterations in population immunity against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant infection and severe disease.
Coronavirus disease 2019 (COVID-19) reinfections and vaccines strengthen the population's resistance to future infections and resultant severe disease. These processes are counterbalanced by immunity waning and immune evasion by novel variants. Estimates of population immunity fluctuations must take into consideration these competing causes.
The evolution of variants with different intrinsic transmissibility and immunity evasive capabilities complicates the modeling of this process. Despite several studies, uncertainty exists regarding the protection offered by various exposure types as well as the rate at which protection diminishes.
About the study
In the present study, researchers assessed the changes in population immunity against SARS-CoV-2 Omicron infection and severe disease predominant in the US between December 2021 and November 2022.
The team gathered Centers for Disease Control and Prevention (CDC) data for immunization and booster coverage at the state and county levels from 2 December 2021 to 9 November 2022. Three vaccination regimes were evaluated.
Firstly, the number of persons who received the primary vaccine series was used as an indicator of vaccination. Second, as an indicator of receiving booster vaccination, the team used the reported number of people who received an additional vaccine dose after completing the primary series. Thirdly, the reported number of persons who had a second booster shot following a primary immunization series and an initial booster dose was used.
The team employed a statistical model that generated a weekly time series of first SARS-CoV-2 infections and reinfections, which accounted for both under-detection and reporting lags, as well as disease progression. This model used the data for reported infection cases and hospitalizations as inputs.
Firstly, the revised model accounts for the reduction of infection-induced immunity and the reduced likelihood that reinfected individuals may proceed to more severe disease states. Secondly, the model was calibrated based on hospitalization reports, not deaths, because of the decreased infection fatality ratio and sparser deaths caused by SARS-CoV-2 over 2022. Thirdly, the infection fatality ratio was revised to reflect the lower death risk associated with Omicron variants.
This evidence was used to establish waning curves for various prior infection and vaccine combinations. The team assumed that immunity wanes at a constant rate; therefore, each fading curve is parameterized by an initial immunity level and an exponential decrease rate. Based on the results presented in the meta-analysis, the team hypothesized that infection-related immunity dropped at the same rate for all non-hybrid exposures and a slower rate for hybrid exposures. For severe disease protection, it was assumed that immunity was reduced by 1% every month and remained constant in the case of hybrid exposures.
In the US, population protection against SARS-CoV-2 Omicron variant infection increased from 22% to 63% between 2 December 2021 and 9 November 2022. In comparison, population immunity to infection by pre-Omicron variants was calculated to be 51% in the fall of 2021. Most of the increased population immunity obtained over the research period was developed during the initial surge in Omicron infections, with estimated infection protection of 57% on 3 March 2022.
As of 9 November 2022, protection observed against COVID-19 infection was between 55% in Mississippi and 69% in Maine. Also, by this date, the team assessed that time trends corresponding to protection against infection in half of the states had begun to decline, which implied that existing immunity was lost faster than the rate at which new immunity was acquired. On 9 November 2022, protection against infection across the US was 40% under the pessimistic waning scenario and 76% under the optimistic waning scenario.
Population immunity observed against a novel Omicron infection was assessed to have increased or remained constant in all states except eight at the end of the surge between December and February and the final phases of the study period. Between 3 March 2022 and 9 November 2022, population immunity declined against a new Omicron infection in over a quarter of the counties.
On a nationwide and state-by-state scale, the protection of populations from severe diseases increased during the study period. Protection against severe COVID-19 caused by a new Omicron infection increased from 61% on 2 December 2021, 84% on 3 March 2022, to 88% by 9 November 2022.
Overall, the study findings showed that in November 2022, protection against SARS-CoV-2 infection, as well as a severe disease, was significantly greater than in December 2021. Despite the high extent of protection observed, a more immune-evading or transmissible variant, behavioral changes, or persistent immunological waning could result in a new wave of SARS-CoV-2 infections.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.