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

Figures & data

Figure 1. A diagrammatic illustration summarizing the structure of the review.

Figure 2. Screening and study selection protocol.

Table 1. Highest and lowest reported vaccine effectiveness (VE, with 95% confidence intervals) values of the COVID-19 vaccines against the Omicron variant in reducing infection as compared to the Delta variant.

Vaccination Status/Vaccine Type	Omicron		Delta
	Lowest	Highest	Lowest	Highest
Partial Vaccination
Pfizer	Adjusted VE 9.5 (−39.8–41.3) 0–13 days^Citation30	Adjusted VE 51.4 (42.7–58.8) 16–17 year old 14–20 days^Citation39	Adjusted VE 30.9 (25.4–36.0) 16–17 year old >105 days^Citation39	Adjusted VE 75.9 (74.3–77.3) 16–17 year old 14–20 days^Citation39
Moderna	Adjusted VE −16.8 (−137.8–42.6) >14 days^Citation30	Adjusted VE 47.9 (43.1–52.3) 0–3 weeks^Citation63	Adjusted VE 56.7 (40.7–68.4)^Citation15	Adjusted VE 60.1 (51.8–66.9) 0–3 weeks^Citation63
Pfizer or Moderna (mRNA)	Unadjusted VE 22 (0–46) 1 dose or 2 doses <7 days^Citation21	Unadjusted VE 22 (0–46) 1 dose or 2 doses <7 days^Citation21	Unadjusted VE 31 (0–72) 1 dose or 2 doses <7 days^Citation21	Unadjusted VE 31 (0–72) 1 dose or 2 doses <7 days^Citation21
AstraZeneca	Adjusted VE : 17.7 (14.3–21.0) 4+weeks^Citation63			Adjusted VE 42.9 (39.8–45.9) 4+weeks^Citation63
Full vaccination
Pfizer	16–17 year olds Adjusted VE −3 (−30–18) ≥150 days^Citation65	12–15 year olds Adjusted VE 83.1 (78.2–86.9) 7–13 days^Citation39	Adjusted VE 53.8 (52.9–54.6) 91–150 days^Citation44	12–15 year olds Adjusted VE 93.2 (81.5–97.5) 7–13 days^Citation39
Moderna	Adjusted VE −39.3 (−61.6 - −20.0) 91–150 days^Citation44	Adjusted VE 75.1 (70.8–78.7) 2–4 weeks^Citation63	Adjusted 61.3 (55.0–66.7) >270 days^Citation15	Adjusted VE 94.5 (90.5–96.9) 2–4 weeks^Citation63
Pfizer or Moderna (mRNA)	Adjusted VE 7 (3–10) ≥ 7 days^Citation16	Adjusted VE 69 (62–75) <2 months^Citation34	Unadjusted VE 42 (23–55) > 7 days^Citation21	Adjusted VE 92 (91–94) <2 months^Citation34
AstraZeneca	Adjusted VE −2.7 (−4.2 - −1.2) >25 weeks^Citation63	Adjusted VE 48.9 (39.2–57.1) 2–4 weeks^Citation63	Adjusted VE 43.5 (42.4–44.5) >25 weeks^Citation63	Adjusted VE 82.8 (74.5–88.4) 2–4 weeks^Citation63
Booster
Pfizer	Adjusted VE 3.6 (0.6–6.5) Vaccination 4 months before infection (VE using 5 months as ref.)^Citation29	Adjusted VE 81 (59–91) 16–17 years old ≥7 days^Citation65	Adjusted VE 81.2 (79.2–82.9) compared to 2 doses) 1–30 days^Citation44	Adjusted VE 97 (96–98) ≥7 days^Citation46
Moderna	Adjusted VE 34.9 (14.6–50.4) > 6 weeks^Citation30	Adjusted 71.6 (69.7–73.4) <60 days^Citation15	Adjusted VE 86.0 (78.1–91.1) >60 days^Citation15	Adjusted VE 97 (95–98) ≥7 days
Pfizer or Moderna (mRNA)	Adjusted VE 31 (−50–68) ≥5 months^Citation34	Adjusted VE 87 (85–88) <2 months^Citation34	Unadjusted VE 65 (42–80) >7 days^Citation21	Adjusted VE 97 (96–97) <2 months^Citation34
AstraZeneca	Adjusted VE 47 (2–70)^Citation43
Mixed Regimens
AstraZeneca + Pfizer Booster	Adjusted VE 39.6 (38.0–41.1) >10 weeks^Citation63	Adjusted VE 58.8 (57.8–59.7) 1 week^Citation63	Adjusted VE 88.1 (86.7–89.3) >10 weeks^Citation63	Adjusted VE 88.7 (88.1–89.2) 1 week^Citation63
AstraZeneca + Moderna Booster	Adjusted VE 60.9 (59.7–62.1) 5–9 weeks^Citation63	Adjusted VE 68.0 (67.0–68.9) 1 week^Citation63	Adjusted VE 91.5 (90.9–92.1) 1 week^Citation63	Adjusted VE 97.0 (96.7–97.3) 2–4 weeks^Citation63
AstraZeneca + AstraZeneca Booster	Adjusted VE 46.7 (34.3–56.7) 5–9 weeks^Citation63	Adjusted VE 57.7 (37.6–71.3) 1 week^Citation63	Adjusted VE 77.1 (55.1–88.3) 1 week^Citation63	Adjusted VE 83.3 (69.7–90.8) 5–9 weeks^Citation63
Moderna + Pfizer Booster	Adjusted VE 64.4 (62.6–66.1) 5–9 weeks^Citation63	Adjusted VE 74.0 (73.1–74.9) 1 week^Citation63	Adjusted VE 93.7 (92.7–94.6) 1 week^Citation63	Adjusted VE 96.6 (96.0–97.1) 2–4 weeks^Citation63
Janssen + Any booster	Adjusted VE 64.3 (61.7–66.8) 1 week^Citation63	Adjusted VE 64.9 (62.3–67.3) 2–4 weeks^Citation63	Adjusted VE 95.5 (92.5–97.3) 1 week^Citation63	Adjusted VE 94.7 (89.3–97.3) 2–4 weeks^Citation63

Table 2. Highest and lowest reported vaccine effectiveness (VE, with 95% confidence intervals) values of the COVID-19 vaccines against the Omicron variant in reducing hospitalization as compared to the Delta and Alpha variants.

Vaccination Status/Vaccine Type	Omicron		Delta		Alpha
	Lowest	Highest	Lowest	Highest	Lowest	Highest
Full vaccination
Pfizer	VE 46 (−15-77) Jan 17-23 (21.0% full vaccine coverage)^Citation45	VE 100 (−189–100) Dec 13–19 (4.7% full vaccine coverage)^Citation45		Adjusted VE 93 (90–94)^Citation47
Moderna		Adjusted VE 84.5 (23.0–96.9)^Citation15		Adjusted VE 99.0 (93.9–99.9)^Citation15
Pfizer or Moderna (mRNA)	Adjusted VE 44 (26–58) ≥14 days^Citation16	Adjusted VE 81 (65–90) <180 days^Citation60	Adjusted VE 75 (70–80) ≥14 days^Citation16	Adjusted VE 94 (92–96) <2 months^Citation34	VE 90 (87–93) Immunocompetent only^Citation22	VE 90 (87–93) Immunocompetent only^Citation22
Booster
Moderna		Adjusted VE 99.2 (76.3–100.0)^Citation15		Adjusted VE (99.7 (96.5–100.0)^Citation15
Pfizer or Moderna (mRNA)	Adjusted VE 78 (67–85) #33 –≥4 months^Citation34	Adjusted VE 91 (88–93) <2 months^Citation34	Adjusted VE 76 (14–93) ≥4 months^Citation34	VE 97 (95–98) Immunocompetent only^Citation22
Janssen		Adjusted VE 85 (54–95) 27–87 days^Citation64

Table 3. Highest and lowest reported vaccine effectiveness (VE, with 95% confidence intervals) values of the COVID-19 vaccines against the Omicron variant in reducing the severity of infection as compared to the Delta variant.

Vaccination Status/Vaccine Type	Omicron		Delta
	Lowest	Highest	Lowest	Highest
Full or partial vaccination
Pfizer or Moderna (mRNA)	Adjusted VE 55 (−106–90) 7–59 days follow up^Citation46	Adjusted VE 86 (12–98) ≥240 days follow up^Citation46	Adjusted VE 94 (84–98) 7–59 days follow up^Citation46	Adjusted 95 (85–99) ≥240 days follow up^Citation46
Full vaccination
Pfizer, Moderna, AstraZeneca or Janssen	Adjusted VE ^#32 (20–43) *37 (12–55) >2 months^Citation37	Adjusted VE ^#57 (32–72) *58 (3–82) <2 months^Citation37	<2 months Adjusted VE ^#82 (76–87) *84 (72–91) <2 months^Citation37	>2 months Adjusted VE ^#82 (80–83) *86 (83–88) >2 months^Citation37
Booster
Pfizer	Adjusted VE 95 (87–98) ≥7 days follow up^Citation46	Unadjusted VE 100 (71.4–100) Post-7d follow-up^Citation19	Adjusted VE 90.1 (80.6–95.0) > 7 week^Citation30	Adjusted VE 99 (98–99) ≥7 days^Citation46
Pfizer or Moderna (mRNA)		Adjusted VE 95 (87–98) ≥7 days follow up^Citation46		Adjusted VE 99 (98–99) ≥7 days follow up^Citation46
AstraZeneca		Adjusted VE 93 (74–98) ≥7 days follow up^Citation46		Adjusted VE 100 (98–100) ≥7 days follow up^Citation46
Pfizer, Moderna, AstraZeneca or Janssen	Adjusted VE ^#85 (80–88) *60 (37–74) >2 months^Citation37	Adjusted VE ^#90 (87–92) *83 (75–89) <2 months^Citation37	Adjusted VE ^#97 (95–98) *97 (92–99) >2 months^Citation37	Adjusted VE ^#98 (98–98) *98 (97–99) <2 months^Citation37

^#Severity defined as ICU admission or death.

*Severity VE is effectiveness to prevent disease progression to invasive mechanical ventilation or death.

Table 4. Highest and lowest reported vaccine effectiveness (VE with 95% confidence intervals) values of the COVID-19 vaccines against the Omicron variant in reducing mortality as compared to the Delta variant.

Vaccination Status/Vaccine Type	Omicron	Delta
Full vaccination
Pfizer or Moderna (mRNA)	Adjusted VE 75 (52–87) ≥14 days^Citation16	Adjusted 93 (85–97) ≥14 days^Citation16
Booster
Pfizer or Moderna (mRNA)	Adjusted VE 94 (85–98) ≥14 days^Citation16	Adjusted VE 96 (88–99) ≥14 days^Citation16

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