To estimate the effectiveness of booster vaccination with either the BNT162b2 or mRNA-1273 vaccines, as compared with that of the two-dose primary series, we used a matched retrospective cohort study design that emulated a target trial.
21 (link),22 (link) The study compared the incidence of symptomatic breakthrough SARS-CoV-2 infection among persons who had received the booster dose more than 7 days previously (the booster cohort) with the incidence among persons who had not yet received a booster dose (the nonbooster cohort). The 7-day cutoff between the administration of the booster and the start of follow-up was informed by earlier studies
22-24 (link) to ensure sufficient time for the buildup of immune protection. A 14-day cutoff was also investigated in a sensitivity analysis.
All persons who had received at least two doses of the BNT162b2 vaccine between January 5, 2021 (the date of the first two-dose BNT162b2 vaccination series in Qatar), and January 26, 2022 (the end of the study), could be included in the eligible cohorts of the study, provided that they had no previous documented infection before the start of follow-up. The same inclusion criteria applied to persons who had received the mRNA-1273 vaccine, but the corresponding dates were January 24, 2021, and January 26, 2022, respectively.
Matching was used to identify a cohort of patients with similar baseline characteristics. Persons in the booster cohort and those in the nonbooster cohort were matched exactly in a 1:1 ratio according to sex, 10-year age group, and nationality to control for known differences in the risk of exposure to SARS-CoV-2 infection in Qatar.
15 (link),25-28 (link) In a previous study that had a similar design, matching according to these factors was shown to provide adequate control of bias arising from differences in this risk. In that study, no meaningful difference between the matched mRNA-1273–vaccinated and BNT162b2-vaccinated cohorts was noted in the incidence of infection in the first 2 weeks after administration of the first dose,
11 (link) as had been expected, given the negligible vaccine protection in this 2-week period.
8 (link),9 (link) Moreover, in previous studies using other designs but the same matching, no meaningful difference was observed between vaccinated persons and unvaccinated persons with respect to the incidence of infection in the first 2 weeks after administration of the first dose.
1 (link),2 (link),20 (link),29 (link)In our study, persons were also matched exactly according to the calendar week of the second-dose vaccination in order to control for the time since vaccination and the waning of vaccine immunity over time.
1 (link),2 (link),10 (link),11 (link) Matching was performed through an iterative process that ensured that each control person in the nonbooster cohort was alive, infection-free, and had not received the third dose of vaccine by the beginning of follow-up. For each matched pair, follow-up began on the eighth day after the person in the booster cohort received the booster dose, provided this day occurred during the wave of infections with the omicron variant (e.g., on or after December 19, 2021). The large exponential-growth phase of this wave of infections started on December 19, 2021, and reached its peak in mid-January 2022.
7 (link),30 Viral whole-genome sequencing of 315 random SARS-CoV-2–positive specimens collected between December 19, 2021, and January 22, 2022, was performed on a GridION sequencing device (Oxford Nanopore Technologies). Of these specimens, 300 (95.2%) were confirmed to be omicron infections and 15 (4.8%) were confirmed to be delta (or B.1.617.2)
5 infections.
7 (link),30 No cases of infection with the delta variant were detected in the viral whole-genome sequencing after January 8, 2022.
Persons in the booster cohort who had received the booster dose at least 7 days before the onset of the wave of omicron infections were followed along with their matched controls in the nonbooster cohort beginning on December 19, 2021. Controls in the nonbooster cohort who received the booster dose at a future date were eligible for recruitment into the booster cohort, provided they were alive and infection-free at the start of follow-up. Accordingly, some persons contributed follow-up time both as persons who had received only a two-dose primary series and as persons who had received a booster, but at different times.
To ensure exchangeability, data on both members of each matched pair were censored once the control received the booster dose.
22 (link) Accordingly, follow-up continued until the first of one of these events: a documented SARS-CoV-2 infection (defined as the first positive PCR or rapid antigen test after the start of follow-up, regardless of the presence of symptoms or the reason for testing [this information was available only for PCR tests]), booster vaccination of the control (with matched pair censoring), death, or the end of study censoring (on January 26, 2022).
Abu-Raddad L.J., Chemaitelly H., Ayoub H.H., AlMukdad S., Yassine H.M., Al-Khatib H.A., Smatti M.K., Tang P., Hasan M.R., Coyle P., Al-Kanaani Z., Al-Kuwari E., Jeremijenko A., Kaleeckal A.H., Latif A.N., Shaik R.M., Abdul-Rahim H.F., Nasrallah G.K., Al-Kuwari M.G., Butt A.A., Al-Romaihi H.E., Al-Thani M.H., Al-Khal A, & Bertollini R. (2022). Effect of mRNA Vaccine Boosters against SARS-CoV-2 Omicron Infection in Qatar. The New England Journal of Medicine.