In a recent Morbidity and Mortality Weekly Report (MMWR), researchers estimated severe acute respiratory syndrome coronavirus 2 (SARS-Co-V2) seroprevalence in the USA (US) blood donors between April 2021 and September 2022. In addition they estimated the incidence of primary infections in the course of the same period stratified by coronavirus disease 2019 (COVID-19) vaccination status.

To this end, they analyzed a nationwide, longitudinal cohort of 142758 blood donors who donated blood two to 3 times in 2020. The Centers for Disease Prevention and Control (CDC) established this cohort in association with Red Cross, Vitalent, Creative Testing Solutions, and Westat.

Study: Estimates of SARS-CoV-2 Seroprevalence and Incidence of Primary SARS-CoV-2 Infections Amongst Blood Donors, by COVID-19 Vaccination Status — United States, April 2021–September 2022. Image Credit: next143 / Shutterstock

Background

From the start of the COVID-19 pandemic to the top of 2022, changes in COVID-19 testing behaviors and test result reporting requirements have hindered authorities from estimating the U.S. SARS-CoV-2 primary infection burden. Nevertheless, this data has public health implications; as an example, it could help discover population subsets at higher risk of infection and devise targeted strategies to mitigate the chance.

Concerning the study

In the current study, researchers randomly chosen one sample per blood donor from the CDC cohort to check for antibodies against the SARS-CoV-2 spike (S) and nucleocapsid (N) antigens in the primary three quarters of 2022 and one quarter of 2021, i.e., April to June 2021. In addition they ascertained whether the donor had received a COVID-19 vaccine at each blood donation visit.

Further, the team weighted the sample data for selection into cohort accounting for 2 aspects, as follows:

i) response in the course of the 4 quarters;

ii) demographic variations between the blood donor and the overall U.S. population. 

Based on each study participant’s antibody testing results and COVID-19 vaccination history, researchers estimated their vaccine-, infection-induced, or hybrid immunity. As well as, they estimated the number of people whose immune status modified between two quarters. This criterion limited this evaluation to 72,748 blood donors, i.e., 51% of cohort strength. An iterative weighting adjustment method, thus, helped obtain weights via stratification and raking combined.

Moreover, the team estimated the seroconversion of anti-N–negative individuals to anti-N–positive individuals across all 4 quarters included on this study. They stratified estimates by race/ethnicity and 4 age bands, 16 to 29, 30 to 49, 50 to 64, and ≥65 years. The cohort comprised people primarily from 4 ethnicities, viz., Asian, Black, Hispanic, and others.

Finally, the team presented seroprevalence and infection rates as weighted means and compared these results using two-sided t-tests. They stratified comparison results by demographics and vaccination status, where a significance level for α was equal to 0.05.

Prevalences of vaccine-induced, infection-induced, and hybrid* immunity^† against SARS-CoV-2 amongst blood donors aged ≥16 years, by age group — United States, April 2021–September 2022

Results

On this study cohort, an estimated 96.4% of people aged 16 years or more had previous infection- or vaccination-induced SARS-CoV-2 antibodies, of which 47.7% had hybrid immunity by the top of September 2022 third quarter; 22.6% of people had developed immunity by natural infection and 26.1% from vaccination.

N antibody seroconversion indicates natural SAR-CoV-2 infection; on this study, it was higher in unvaccinated individuals relative to vaccinated people, showing that vaccination provided some protection against SARS-CoV-2 infection. It is also because vaccinated and unvaccinated individuals have different prevention behaviors, including face masking and social distancing. 

In the course of the period of dominance of the SARS-CoV-2 Omicron variant, the relative difference in infection rates narrowed down, most probably because vaccine-induced protection wanes more rapidly as time elapsed post-vaccination increases. Also, Omicron evades all elicited immunity to cause breakthrough infections. Nevertheless, by late 2022, the difference in infection rates also narrowed down between vaccinated and unvaccinated individuals because each adopted nearly similar prevention behaviors.

Likewise, the incidence of index SARS-CoV-2 infection(s) was higher amongst younger vs. older individuals and lower amongst Asian vs. other races and ethnicities; nonetheless, these differences amongst all evaluated groups also narrowed down with time. Intriguingly, the prevalence of hybrid immunity was the very best amongst Black and Hispanic populations vs. Asians and White people.

Conclusions

On this report, researchers detected that almost 50% of individuals aged ≥16 years with immunity had hybrid immunity, and two-thirds contracted a natural infection. Each infection-induced and hybrid immunity increased in the course of the study duration, however the prevalence of hybrid immunity remained low amongst adults aged 65 years or older. These results showed the positive, useful effects of more vaccination coverage and immediate availability of COVID-19 vaccines for the elderly population within the US and worldwide.

Furthermore, the older adults, i.e., people aged 65 years or more, adhered to beneficial behavioral practices to avoid SARS-CoV-2 infection greater than the opposite three age groups monitored on this study. Nonetheless, as suggested by the current study results, for people at high risk of severe SARS-CoV-2 infection, even others, it’s crucial to stay updated with beneficial COVID-19 vaccination, including not less than one bivalent dose.