Effectiveness of the pre-Omicron COVID-19 vaccines against Omicron in reducing infection, hospitalization, severity, and mortality compared to Delta and other variants: A systematic review

Abstract

Despite widespread mass rollout programs, the rapid spread of the SARS-CoV-2 Omicron variant called into question the effectiveness of the existing vaccines against infection, hospitalization, severity, and mortality compared to previous variants. This systematic review summarizes and compares the effectiveness of the COVID-19 vaccines, with respect to the above outcomes in adults, children, and adolescents. A comprehensive literature search was undertaken on several databases. Only 51 studies met our inclusion criteria, revealing that the protection from primary vaccination against Omicron infection is inferior to protection against Delta and Alpha infections and wanes faster over time. However, mRNA vaccine boosters were reported to reestablish effectiveness, although to a lower extent against Omicron. Nonetheless, primary vaccination was shown to preserve strong protection against Omicron-associated hospitalization, severity, and death, even months after last dose. However, boosters provide more robust and longer-lasting protection against hospitalizations due to Omicron as compared to only primary series.

A Phase II Study Integrating a Single-Blind Safety Phase with a Double-Blind, Placebo-Controlled Randomized Phase, Assessing Single-Dose Intramuscular or Intranasal Administration to Evaluate the Safety and Immunogenicity of the Recombinant Vaccine Against COVID-19 (AVX/COVID-12 “Patria”) Based on an Active Newcastle Disease Viral Vector as a Heterologous Booster in Subjects with Evidence of Previous Immunity to SARS-CoV-2

“…Real-world evidence indicates that first-generation vaccines, utilizing the spike protein from the original Wuhan-1 strain, exhibit high effectiveness against the severity of infection (intensive care unit (ICU) admission or death) caused by various subvariants of Omicron. For instance, a full vaccination scheme with a booster from Moderna shows 95% effectiveness, Pfizer demonstrates 95-100% (22,23), while AstraZeneca or Janssen exhibits 85-90% effectiveness (23). Additionally, the mRNA vaccine BNT162b2 has been reported to be 84.9% effective in preventing intensive care unit admission for COVID-19 in children and 95% in adolescents (24).…”

The Course of COVID-19 and Long COVID: Identifying Risk Factors among Patients Suffering from the Disease before and during the Omicron-Dominant Period

“…A systematic review by Paul et al indicated that primary vaccinations were short-lasting and less protective against Omicron relative to the Delta and Alpha variants. Although booster doses reestablished the vaccination’s effectiveness, they did so to a lower extent against Omicron [ 53 ]. Another comprehensive approach by Arabi et al certified that earlier infection protects against Omicron, even though the degree of immunity was much lower relative to Delta.…”

The relative prevalence of the Omicron variant within SARS-CoV-2 infected cohorts in different countries: A systematic review

“…Paul et al 92 reported that the pre-Omicron mRNA vaccine boosters were reported to reestablish effectiveness, although to a lower extent against Omicron. Nonetheless, primary vaccination was shown to preserve strong protection against Omicron-associated hospitalization, severity, and death, even months after last dose.…”