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In persons who had received the BNT162b2 vaccine in Qatar, the incidence of infection with the omicron variant after 35 days of observation was 2.4% among those who had received three doses and 4.5% among those who were vaccinated but not boosted; among those who had received the mRNA-1273 vaccine, the incidence was 1.0% with a boost and 1.9% without.

In a test-negative, case–control study involving more than 900,000 participants in Qatar, vaccine effectiveness peaked at 77.5% in the first month after the second dose. The effectiveness fell thereafter to as low as 20% in months 5 through 7 after vaccination, but protection against serious Covid-19 remained greater than 90% for at least 6 months.

A study in Qatar assessed the effectiveness of previous infection, vaccination, and both against symptomatic SARS-CoV-2 caused by omicron BA.1 and BA.2 and against severe, critical, or fatal Covid-19.

Using a national Covid-19 database in Qatar, investigators found that previous SARS-CoV-2 infection provided protection against subsequent reinfection that ranged from 85% to 92% for the alpha, beta, and delta strains and was approximately 60% protective against the omicron variant. Previous infection also appeared to protect against severe disease, hospitalization, and death.

With the global expansion of the highly transmissible SARS-CoV-2 Delta (B.1.617.2) variant, we conducted a matched test-negative case–control study to assess the real-world effectiveness of COVID-19 messenger RNA vaccines against infection with Delta in Qatar’s population. BNT162b2 effectiveness against any, symptomatic or asymptomatic, Delta infection was 45.3% (95% CI, 22.0–61.6%) ≥14 d after the first vaccine dose, but only 51.9% (95% CI, 47.0–56.4%) ≥14 d after the second dose, with 50% of fully vaccinated individuals receiving their second dose before 11 May 2021. Corresponding mRNA-1273 effectiveness ≥14 d after the first or second dose was 73.7% (95% CI, 58.1–83.5%) and 73.1% (95% CI, 67.5–77.8%), respectively. Notably, effectiveness against Delta-induced severe, critical or fatal disease was 93.4% (95% CI, 85.4–97.0%) for BNT162b2 and 96.1% (95% CI, 71.6–99.5%) for mRNA-1273 ≥ 14 d after the second dose. Our findings show robust effectiveness for both BNT162b2 and mRNA-1273 in preventing Delta hospitalization and death in Qatar’s population, despite lower effectiveness in preventing infection, particularly for the BNT162b2 vaccine. mRNA COVID-19 vaccines are highly effective at preventing severe outcomes and death caused by the SARS-CoV-2 Delta variant (B.1.617.2) in Qatar despite substantially lower effectiveness at blocking infection.

The SARS-CoV-2 pandemic continues to be a global health concern. The mRNA-1273 (Moderna) vaccine was reported to have an efficacy of 94.1% at preventing symptomatic COVID-19 due to infection with ‘wild-type’ variants in a randomized clinical trial. Here, we assess the real-world effectiveness of this vaccine against SARS-CoV-2 variants of concern, specifically B.1.1.7 (Alpha) and B.1.351 (Beta), in Qatar, a population that comprises mainly working-age adults, using a matched test-negative, case-control study design. We show that vaccine effectiveness was negligible for 2 weeks after the first dose, but increased rapidly in the third and fourth weeks immediately before administration of a second dose. Effectiveness against B.1.1.7 infection was 88.1% (95% confidence interval (CI): 83.7–91.5%) ≥14 days after the first dose but before the second dose, and was 100% (95% CI: 91.8–100.0%) ≥14 days after the second dose. Analogous effectiveness against B.1.351 infection was 61.3% after the first dose (95% CI: 56.5–65.5%) and 96.4% after the second dose (95% CI: 91.9–98.7%). Effectiveness against any severe, critical or fatal COVID-19 disease due to any SARS-CoV-2 infection (predominantly B.1.1.7 and B.1.351) was 81.6% (95% CI: 71.0–88.8%) and 95.7% (95% CI: 73.4–99.9%) after the first and second dose, respectively. The mRNA-1273 vaccine is highly effective against B.1.1.7 and B.1.351 infections, whether symptomatic or asymptomatic, and against any COVID-19 hospitalization and death, even after a single dose. A matched test-negative, case-control study using real-world data from a predominantly working-age population demonstrates efficacy of the mRNA-1273 vaccine to be 100% and 96.4% against the B.1.1.7 (Alpha) and B.1.351 (Beta) SARS-CoV-2 variants of concern, respectively.

SARS-CoV-2 Omicron BA.1 and BA.2 subvariants are genetically divergent. We conducted a matched, test-negative, case-control study to estimate duration of protection of the second and third/booster doses of mRNA COVID-19 vaccines against BA.1 and BA.2 infections in Qatar. BNT162b2 effectiveness was highest at 46.6% (95% CI: 33.4–57.2%) against symptomatic BA.1 and at 51.7% (95% CI: 43.2–58.9%) against symptomatic BA.2 infections in the first three months after the second dose, but declined to ~10% or below thereafter. Effectiveness rebounded to 59.9% (95% CI: 51.2–67.0%) and 43.7% (95% CI: 36.5–50.0%), respectively, in the first month after the booster dose, before declining again. Effectiveness against COVID-19 hospitalization and death was 70–80% after the second dose and >90% after the booster dose. mRNA-1273 vaccine protection showed similar patterns. mRNA vaccines provide comparable, moderate, and short-lived protection against symptomatic BA.1 and BA.2 Omicron infections, but strong and durable protection against COVID-19 hospitalization and death. The SARS-CoV-2 Omicron variant has subvariants with divergent properties but relative vaccine effectiveness has not been characterized. Here, the authors show that mRNA vaccine effectiveness is similar for the subvariants BA.1 and BA.2, with a decline three months after the second dose and increase after the booster.

The epidemiology of the SARS-CoV-2 B.1.1.7 (or Alpha) variant is insufficiently understood. This study's objective was to describe the introduction and expansion of this variant in Qatar and to estimate the efficacy of natural infection against reinfection with this variant. Reinfections with the B.1.1.7 variant and variants of unknown status were investigated in a national cohort of 158,608 individuals with prior PCR-confirmed infections and a national cohort of 42,848 antibody-positive individuals. Infections with B.1.1.7 and variants of unknown status were also investigated in a national comparator cohort of 132,701 antibody-negative individuals. B.1.1.7 was first identified in Qatar on 25 December 2020. Sudden, large B.1.1.7 epidemic expansion was observed starting on 18 January 2021, triggering the onset of epidemic's second wave, 7 months after the first wave. B.1.1.7 was about 60% more infectious than the original (wild-type) circulating variants. Among persons with a prior PCR-confirmed infection, the efficacy of natural infection against reinfection was estimated to be 97.5% (95% CI: 95.7% to 98.6%) for B.1.1.7 and 92.2% (95% CI: 90.6% to 93.5%) for variants of unknown status. Among antibody-positive persons, the efficacy of natural infection against reinfection was estimated to be 97.0% (95% CI: 92.5% to 98.7%) for B.1.1.7 and 94.2% (95% CI: 91.8% to 96.0%) for variants of unknown status. A main limitation of this study is assessment of reinfections based on documented PCR-confirmed reinfections, but other reinfections could have occurred and gone undocumented. In this study, we observed that introduction of B.1.1.7 into a naïve population can create a major epidemic wave, but natural immunity in those previously infected was strongly associated with limited incidence of reinfection by B.1.1.7 or other variants.

Epidemiologic data from Qatar show that any previous SARS-CoV-2 infection was 35% effective in preventing reinfection with omicron BA.4 and BA.5 subvariants and previous omicron infection was 76% effective.

Long-term effectiveness of COVID-19 mRNA boosters in populations with different previous infection histories and clinical vulnerability profiles is inadequately understood. We aimed to investigate the effectiveness of a booster (third dose) vaccination against SARS-CoV-2 infection and against severe, critical, or fatal COVID-19, relative to that of primary-series (two-dose) vaccination over a follow-up duration of 1 year. This observational, matched, retrospective, cohort study was done on the population of Qatar in people with different immune histories and different clinical vulnerability to infection. The source of data are Qatar's national databases for COVID-19 laboratory testing, vaccination, hospitalisation, and death. Associations were estimated using inverse-probability-weighted Cox proportional-hazards regression models. The primary outcome of the study is the effectiveness of COVID-19 mRNA boosters against infection and against severe COVID-19. Data were obtained for 2 228 686 people who had received at least two vaccine doses starting from Jan 5, 2021, of whom 658 947 (29·6%) went on to receive a third dose before data cutoff on Oct 12, 2022. There were 20 528 incident infections in the three-dose cohort and 30 771 infections in the two-dose cohort. Booster effectiveness relative to primary series was 26·2% (95% CI 23·6–28·6) against infection and 75·1% (40·2–89·6) against severe, critical, or fatal COVID-19, during 1-year follow-up after the booster. Among people clinically vulnerable to severe COVID-19, effectiveness was 34·2% (27·0–40·6) against infection and 76·6% (34·5–91·7) against severe, critical, or fatal COVID-19. Effectiveness against infection was highest at 61·4% (60·2–62·6) in the first month after the booster but waned thereafter and was modest at only 15·5% (8·3–22·2) by the sixth month. In the seventh month and thereafter, coincident with BA.4/BA.5 and BA.2·75* subvariant incidence, effectiveness was progressively negative albeit with wide CIs. Similar patterns of protection were observed irrespective of previous infection status, clinical vulnerability, or type of vaccine (BNT162b2 vs mRNA-1273). Protection against omicron infection waned after the booster, and eventually suggested a possibility for negative immune imprinting. However, boosters substantially reduced infection and severe COVID-19, particularly among individuals who were clinically vulnerable, affirming the public health value of booster vaccination. The Biomedical Research Program and the Biostatistics, Epidemiology, and the Biomathematics Research Core (both at Weill Cornell Medicine-Qatar), Ministry of Public Health, Hamad Medical Corporation, Sidra Medicine, Qatar Genome Programme, and Qatar University Biomedical Research Center.