Risk Of Infection, Hospitalisation, And Death Up To 9 Months After A Second Dose Of COVID-19 Vaccine: A Retrospective, Total Population Cohort Study in Sweden

Contact: Audrey Hu Whatsapp/hp: 0086 13880143964 Email: audrey.hu@wecistanche.com

Peter Nordström, Marcel Ballin, Anna Nordström

Department of Community Medicine and Rehabilitation, Unit of Geriatric Medicine (Prof P Nordström Ph.D., M Ballin MSc, A Nordström Ph.D.)

Department of Public Health and Clinical Medicine, Section of Sustainable Health (M Ballin, A Nordström), Umeå University, Umeå, Sweden;

School of Sport Sciences, UiT the Arctic University of Norway, Tromsø, Norway (A Nordström)

Summary

Background: Vaccine effectiveness against COVID-19 beyond 6 months remains incompletely understood. We aimed to investigate the effectiveness of COVID-19 vaccination against the risk of infection, hospitalization, and death during the first 9 months after vaccination for the total population of Sweden. Methods: This retrospective, total population cohort study was done using data from Swedish nationwide registers. The cohort comprised all individuals vaccinated with two doses of ChAdOx1 n CoV-19, mRNA-1273, or BNT162b2, and matched unvaccinated individuals, with data on vaccinations and infections updated until Oct 4, 2021. Two outcomes were evaluated. The first was SARS-CoV-2 infection of any severity from Jan 12 to Oct 4, 2021. The second was severe COVID-19, defined as hospitalization for COVID-19 or all-cause 30-day mortality after confirmed infection, from March 15 to Sept 28, 2021. Findings: Between Dec 28, 2020, and Oct 4, 2021, 842974 individuals were fully vaccinated (two doses) and were matched (1:1) to an equal number of unvaccinated individuals (total study cohort n=1685 948). For the outcome, SARS-CoV-2 infection of any severity, the vaccine effectiveness of BNT162b2 waned progressively over time, from 92% (95% CI 92 to 93; p<0·001) at 15–30 days, to 47% (39 to 55; p<0·001) at 121–180 days, and to 23% (–2 to 41; p=0·07) from day 211 onwards. Waning was slightly slower for mRNA-1273, with vaccine effectiveness of 96% (94 to 97; p<0·001) at 15–30 days and 59% (18 to 79; p=0·012) from day 181 onwards. Waning was also slightly slower for heterologous ChAdOx1 nCoV-19 plus an mRNA vaccine, for which vaccine effectiveness was 89% (79 to 94; p<0·001) at 15–30 days and 66% (41 to 80; p<0·001) from day 121 onwards. By contrast, vaccine effectiveness for homologous ChAdOx1 nCoV-19 vaccine was 68% (52 to 79; p<0·001) at 15–30 days, with no detectable effectiveness from day 121 onwards (–19% [–98 to 28]; p=0·49). For the outcome of severe COVID-19, vaccine effectiveness waned from 89% (82 to 93; p<0·001) at 15–30 days to 64% (44 to 77; p<0·001) from day 121 onwards. Overall, there was some evidence for lower vaccine effectiveness in men than in women and in older individuals than in younger individuals. Interpretation: We found progressively waning vaccine effectiveness against SARS-CoV-2 infection of any severity across all subgroups, but the rate of waning differed according to vaccine type. With respect to severe COVID-19, vaccine effectiveness seemed to be better maintained, although some waning became evident after 4 months. The results strengthen the evidence-based rationale for the administration of a third vaccine dose as a booster.

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Introduction

Randomized clinical trials have shown high efficacy of the BNT162b2 (Pfizer-BioNTech),1 mRNA-1273 (Moderna),2 and ChAdOx1 n CoV-19 (Oxford-AstraZeneca) COVID-19 vaccines,3,4 and observational studies have estimated high real-world effectiveness.5–8 However, reports on breakthrough infections9 and waning immunity10–14 have raised concerns regarding the duration of protection.

With respect to severe COVID-19 outcomes such as hospitalization or death, follow-ups of clinical trials showed that after 4 months the efficacy of BNT162b2 was about 84%15 and the efficacy of mRNA-1273 was about 92%,16 with similar results reported by the US Centers for Disease Control and Prevention.17 Observational studies from the USA and Qatar also showed that the effectiveness of BNT162b2 against hospitalization and death persisted up to 6 months,18,19 whereas preliminary data from the UK indicate a slight warning, especially in older adults and of ChAdOx1 nCoV-19 compared with BNT162b2.20 In terms of ChAdOx1 n CoV-19, another observational study reported waning effectiveness against hospitalization and death within 3 months in Brazil and Scotland.21 Altogether, although evidence suggests that vaccine effectiveness against severe COVID-19 is relatively well maintained, the data are inconsistent. Similarly, the duration of protection against SARS-CoV-2 infection of any severity is unclear. After 4–5 months of follow-up, the effectiveness of BNT162b2 has been estimated as greater than 80% in one study,15 around 50% in two other studies,19,20 and as low as around 20% in a study from Qatar.18 For ChAdOx1 n CoV-19, preliminary data from the UK suggest about 50% remaining effectiveness after 5 months of follow-up,20 whereas a published study showed that the effectiveness was down to about 50% in Scotland and 60% in Brazil after about 4 months.21

The different results in recent studies might relate to several factors, such as the evaluations of vaccines that might have different long-lasting effects,16,18–20 different ages of the participants,18 varying and relatively short follow-up times,15,16,22 different patterns of risk compensation in the populations, different severities and definitions of infections included as outcomes, and variations in infection pressure and variant exposure during follow-up. Collectively, vaccine effectiveness beyond 6 months remains incompletely understood. In this study, we investigated the effectiveness of COVID-19 vaccination against the risk of infection, hospitalization, and death during the first 9 months after vaccination for the total population of Sweden.

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Methods

Study design and participants

This retrospective, total population cohort study was done in Sweden. We included all individuals (n=3 640 421) vaccinated with at least one dose of any COVID-19 vaccine (ChAdOx1 nCoV-19, BNT162b2, or mRNA-1273) in Sweden until May 26, 2021, and all individuals with a documented SARS-CoV-2 infection until May 24, 2021 (n=1 331 989). Each individual was then matched (1:1) by Statistics Sweden, the national agency for statistics, to one randomly sampled individual from the total population of Sweden on birth year, sex, and municipality. In total, the cohort (vaccinated, those with documented infection, and matches) consisted of 5 833 003 individuals. This cohort was updated with respect to data on vaccinations and documented infections until Oct 4, 2021. From this cohort, each individual who was vaccinated with two doses, with no documented SARS-CoV-2 infection and alive within 14 days of vaccination, was matched (1:1) to one randomly sampled individual from the rest of the cohort on birth year and sex. Baseline for both individuals in each matched pair was set to the date of the second dose of vaccine in the vaccinated individual. Matched individuals were excluded if they received the first dose of vaccine, had a documented previous SARS-CoV-2 infection, or died within 14 days of baseline, whereby a new individual was searched from the remaining total cohort. This procedure was repeated five times. Data on individuals vaccinated against COVID-19 and data on documented SARS-CoV-2 infections were collected from the Swedish Vaccination Register and the Smet register, respectively, both of which are managed by the Public Health Agency of Sweden.23,24 All healthcare providers in Sweden are obliged to report to these registers according to Swedish law, with 100% coverage of the total population.

In the main cohort, cases of SARS-CoV-2 infections of any severity were recorded from Jan 12 to Oct 4, 2021, and cases of severe COVID-19 were recorded from March 15 to Sept 28, 2021. From the main cohort, we also formed four subcohorts according to specific vaccine types (BNT162b2, mRNA-1273, ChAdOx1 n CoV-19) and schedule (heterologous ChAdOx1 nCoV-19 plus an mRNA vaccine).

We also formed a second cohort to be used in a sensitivity analysis. This second cohort was formed using less strict matching criteria to increase the size of the cohort. In this dataset, each vaccinated individual was matched to the rest of the cohort on age only, with an allowance of a 5-year difference in age within each pair. This procedure was repeated ten times and one matched unvaccinated individual could be paired with several vaccinated individuals.

This study was approved by the Swedish Ethical Review Authority (number 495/2021), which waived the requirement of obtaining informed consent given the retrospective study design.

Exposures and outcomes

The exposure variable was vaccination status (vaccinated with two doses vs unvaccinated). Vaccination status was defined according to each specific vaccine schedule, as well as a composite variable (any vaccine). There were two outcomes of the study. The first was SARS-CoV-2 infection of any severity until Oct 4, 2021. In 94·4% of cases, infections were confirmed using PCR, and in 4·8% by sequencing, according to the Smi-Net register. The second outcome was a composite endpoint of severe COVID-19, defined as inpatient hospitalization with COVID-19 as the main diagnosis, and all-cause mortality within 30 days after confirmed SARS-CoV-2 infection. This outcome was collected until Sept 28, 2021. Data on patients admitted to the hospital were collected from the Swedish National Inpatient Register using the International Classification of Diseases version 10 (ICD-10), code U071, and Statistics Sweden provided data on mortality. All outcomes were collected from more than 14 days after baseline.

Covariates

From Statistics Sweden, we obtained information on whether individuals were born in Sweden or not, birth year, birth month, and sex for all individuals. From Statistics Sweden, we also obtained individual-level data on the highest education during 2019. Individual-level data regarding diagnoses, prescription medications, and homemaker services were obtained from national registers managed by the Swedish National Board of Health and Welfare. Homemaker services include domestic services provided to individuals (primarily older individuals) who live at home but need help with shopping, cleaning, meal preparation, and similar tasks. From the Swedish National Inpatient Register and National Outpatient Register for specialist care, diagnoses from 1998 and 2001 and later were obtained using ICD-10 codes. Prescription medications from 2018 and later were obtained from the Prescribed Drug Register using Anatomic Therapeutic Chemical classification system codes. These three registers are complete for all specialist care and medications prescribed in Sweden for the years selected. The diagnoses and medications selected as covariates for this study were selected a priori based on the results from a previous nationwide study.25 Definitions of comorbidities are shown in the appendix (p 2).

Statistical analysis

Hazards overtime for the outcome of SARS-CoV-2 infection of any severity, based on exposure status (vaccinated vs unvaccinated), are shown using proportional hazards models with 95% CIs and restricted cubic splines. To compare the risk of the outcomes based on exposure status (vaccinated vs unvaccinated), Cox regression was used to calculate hazard ratios (HRs). To adjust for the matched samples, 95% CIs were estimated using robust SEs by the variance-covariance matrix of the estimator's procedure and robust option in Stata. To formally test whether the associations were time-dependent, Schoenfeld’s residuals were evaluated using estate test command (Stata software). Given that the test indicated that the proportional hazard assumption was violated (χ² = 3184·25; p<0·001) in the main analyses, the associations were evaluated in time intervals. The first model was adjusted for age and baseline date (date of the second dose of vaccine) to adjust for variations in infection pressure during follow-up. The second model included the additional covariates sex, homemaker service (yes or no), education (six categories), whether the individual was born in Sweden or not, and eight diagnoses at baseline (yes or no). The HR was used to calculate vaccine effectiveness using the following formula: vaccine effectiveness=(1 – HR) × 100%. To investigate whether vaccine effectiveness was influenced by the prespecified covariates, interaction analyses were done, using product terms created by multiplying the variable coding for vaccination status at baseline (vaccinated vs unvaccinated) by each respective covariate, which was added to the fully adjusted Cox model. Given that the interaction terms were highly significant (p<0·001) for age, sex, homemaker service, and all diagnoses at baseline except asthma, vaccine effectiveness was also estimated in subgroups according to these covariates. Follow-up time in days was counted until the date of the confirmed outcome, date of first vaccination after baseline among unvaccinated individuals, death, or end of possible follow-up time (described earlier), whichever occurred first. All analyses were done in SPSS (version 27.0 for Mac), and Stata (version 16.1 for Mac). A two-sided p-value less than 0·05 or HR with 95% CIs not crossing one were considered significant.

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Results

Between Dec 28, 2020, and Oct 4, 2021, 842974 individuals were fully vaccinated (two doses) and were matched (1:1) to an equal number of unvaccinated individuals. Thus, the total study cohort comprised 842974 pairs (n=1685948; figure 1). The mean date for the second dose of vaccine in the vaccinated group according to each vaccine schedule is shown in table 1, together with baseline characteristics. Compared with unvaccinated individuals, vaccinated individuals more often had homemaker service, were more often born in Sweden, had more comorbidities, and had a higher level of education at baseline (p<0·001 for all; table 1). Similar differences were evident between vaccinated and unvaccinated individuals in the different vaccine subcohorts. SARS-CoV-2 variants sequenced in Sweden during the study period are shown in the appendix (p 2).

During a median follow-up of 108 days (IQR 69–145), a SARS-CoV-2 infection was confirmed in 27 918 individuals, of whom 6147 were vaccinated (4·9 infections per 100 000 person-days) and 21 771 were unvaccinated (31·6 infections per 100 000 person-days). The vaccine effectiveness associated with two doses of any vaccine peaked at 15–30 days (92% [95% CI 91 to 93]; p<0·001) and declined marginally at 31–60 days (89% [88 to 89]; p<0·001; table 2, figure 2). From thereon, the waning became more pronounced, and from day 211 onwards there was no remaining detectable vaccine effectiveness (23% [–2 to 41]; p=0·07).

The estimated vaccine effectiveness was influenced significantly by vaccine type, age, sex, homemaker service, and all diagnoses at baseline (pinteraction<0·001 for all), except asthma (pinteraction=0·86). At 61–120 days, vaccine effectiveness declined to 50% (95% CI 30 to 64; p<0·001) in individuals aged 80 years or older, and to 61% (47–72; p<0·001) in individuals with homemaker service (table 3). With respect to sex, there was no detectable vaccine effectiveness in men (17% [95% CI –13 to 40]; p=0·23) from day 181 onwards, whereas it remained in women (34% [22 to 45]; p<0·001). With respect to vaccine type, vaccine effectiveness waned progressively for all vaccines during follow-up, but at different speeds (table 2). The vaccine effectiveness of BNT162b2 was 92% (95% CI 92 to 93; p<0·001) at 15–30 days, 47% (39 to 55; p<0·001) at 121–180 days, and 23% (–2 to 41; p=0·07) from day 211 onwards. Waning was slightly slower for mRNA-1273, with vaccine effectiveness of 96% (94 to 97; p<0·001) at 15–30 days and 59% (18 to 79; p=0·012) from day 181 onwards. Waning was also slightly slower for heterologous ChAdOx1 nCoV-19 plus mRNA vaccine schedules, with vaccine effectiveness of 89% (79 to 94; p<0·001) at 15–30 days and 66% (41 to 80; p<0·001) from day 121 onwards. By contrast, vaccine effectiveness for homologous ChAdOx1 n CoV-19 was 68% (52 to 79; p<0·001) at 15–30 days, with no detectable effectiveness from day 121 onwards (–19% [95% CI –98 to 28]; p=0·49).

During a median follow-up of 124 days (IQR 98–208), there were 277 cases of COVID-19 hospitalization or death among vaccinated individuals (0·23 hospitalizations or deaths per 100000 person-days) and 825 cases among unvaccinated individuals (1·20 hospitalizations or deaths per 100000 person-days; appendix pp 3, 7). The vaccine effectiveness associated with two doses of any vaccine was 89% (95% CI 83 to 93; p<0·001) at 15–30 days, which declined to 64% (44 to 77; p<0·001) from day 121 onwards (appendix p 3).

In a sensitivity analysis using less strict matching criteria, a second matched cohort (1 983 315 matched pairs; n=3 996 630) more than twice the size of the original cohort was created. The mean age of vaccinated individuals was 5 years higher in the second cohort than in the main cohort, whereas all other characteristics were similar between the cohorts (appendix p 3). In this larger cohort, the waning vaccine effectiveness was confirmed with respect to a SARS-CoV-2 infection of any severity (appendix p 4), including the different rate of waning for different vaccine schedules (appendix p 5). In addition, it was confirmed that vaccine effectiveness was better maintained against the outcome of severe COVID-19 (appendix p 6), than against SARS-CoV-2 infection of any severity (appendix p 4).

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Discussion

This study showed progressive waning vaccine effectiveness against SARS-CoV-2 infection of any severity during up to 9 months of follow-up. In the main cohort, the estimated vaccine effectiveness was more than 90% in the first month, with a progressive waning starting soon thereafter, ultimately resulting in nondetectable vaccine effectiveness after 7 months. Vaccine effectiveness waned across all subgroups, although differently according to vaccine schedule and type. Vaccine effectiveness with respect to the risk of COVID-19 hospitalization or death seemed to be better maintained than effectiveness against infection, although some waning became evident after 4 months. Overall, there was also some evidence suggesting lower vaccine effectiveness in men than in women and in older individuals than in younger individuals.

Waning vaccine effectiveness against SARS-CoV-2 infection has previously been reported in preliminary observational studies from the UK and in published observational studies from the USA and Qatar,18–20 whereas follow-up studies of clinical trials show high remaining efficacy of both BNT162b2 after 4 months,15 and mRNA-1273 after more than 4 months.16 Our data add to these previous studies with a follow-up time of up to 9 months, about 28000 confirmed SARS-CoV-2 infections in the main cohort, and the evaluation of four different vaccine schedules in a real-world setting. Overall, our results showed notable waning vaccine effectiveness against SARS-CoV-2 infection of any severity across all subgroups, although with higher remaining vaccine effectiveness for mRNA-1273 and for heterologous vaccine schedules. The latter finding is of particular interest and is supported by clinical trials showing superior vaccine-elicited immunogenicity from heterologous vaccine schedules.26,27 Our finding is in addition to previous observational studies estimating high vaccine effectiveness of heterologous schedules in the short term.28,29 By contrast, we were not able to detect any remaining vaccine effectiveness against SARS-CoV-2 infection of any severity from homologous ChAdOx1 n CoV-19 vaccination after more than 4 months. This finding contradicts preliminary evidence from the UK,20 but is in line with a recent study reporting waning vaccine effectiveness for this vaccine against both SARS-CoV-2 infection and severe COVID-19 in Brazil and Scotland within 3 months of the second dose.21 The different estimates of vaccine effectiveness in all of these studies could be influenced by several factors—eg, different patterns of risk compensation, undiagnosed previous infections in individuals used as controls, varying follow-up times, the prevalence of risk factors that reduce the immune response to vaccination, the severity, and definition of infections included as outcomes, variations in infection pressure and SARS-CoV-2 variants during follow-up, and different age of the studied populations.

In the present study, vaccine effectiveness against severe COVID-19 was better maintained than against SARS-CoV-2 infections of any severity, although some waning was evident after more than 4 months. These results were confirmed in a sensitivity analysis done in a second, even larger cohort, and have some support from preliminary data originating from the UK.20 In the same report, waning seemed greater in individuals belonging to a clinically vulnerable group and in older adults,20 as indicated also from the sensitivity analysis in the present study. A reasonable explanation of waning effectiveness predominantly in older adults would be that the adaptive immune response mediated by B cells that produce antibodies, as well as T cells is impaired with older age.30 In support, one of the risk factors associated with lower vaccine effectiveness in the present study was older age. Among other risk factors for lower vaccine effectiveness was the male sex. Although there has been no previous study reporting waning vaccine effectiveness according to sex, these findings are supported by studies showing lower vaccine-elicited immunogenicity along with a more rapid decline in neutralizing antibody titers in men compared within women.13,31

The results of our study have important clinical implications, as they strengthen the evidence-based rationale for the administration of a third vaccine dose as a booster, especially to specific high-risk populations. Recent preliminary phase 3 data from Pfizer-BioNTech show that a third dose of BNT162b2, administered a median of 11 months after the second dose, had 95·6% efficacy (95% CI 89·3–98·6) against symptomatic COVID-19 compared with those who had only received two primary doses, with consistent results irrespective of age, sex, and comorbidities.32 In addition, data from an Israeli observational study showed that individuals who received the third dose of BNT162b2 had a reduced rate of infections and hospitalizations compared with individuals given two doses.33 Currently, many countries are recommending a third vaccine dose as a booster to select populations at increased risk of severe COVID-19. The implication of the results from the present study and previous studies is that older individuals and individuals with known suboptimal or waning vaccine-elicited immunogenicity should be prioritized for booster doses because these individuals also are at the highest risk for severe COVID-19 manifestations if infected.

Other than the observational design, the present study has some limitations to consider. Although we adjusted our analyses for several potential confounders, the possibility of residual and unmeasured confounding remains, including a higher risk of selection bias in unvaccinated individuals with longer follow-up times. Moreover, although we excluded all individuals with a documented previous infection, some individuals with a previous asymptomatic infection are likely to have been included in the analyses. Furthermore, the SARS-CoV-2 infections registered in the Smi-Net register included infections of any severity, and the definition of severe COVID-19 included death from any cause within 30 days after a confirmed infection. More strict definitions might have increased the estimates of vaccine effectiveness for both outcomes. However, it should be noted that vaccine effectiveness was greater than 90% early after vaccination. Finally, the follow-up in the present study was completed before the emergence of the recent omicron (B.1.1.529) variant of SARS-CoV-2. This study also has several important strengths. First, the results were confirmed in sensitivity analyses based on a second cohort where less strict matching criteria were used. Second, vaccinated individuals had received different types and combinations of vaccines, allowing us to investigate how this differentially affected vaccine effectiveness and duration of vaccine protection in a real-world setting. Third, all the registers used to obtain data on COVID-19 cases, vaccinations, hospitalizations, and deaths have nationwide coverage and zero loss to follow-up. This reduces the risk of misclassification of unvaccinated individuals included in the analyses. Using these registers, we were also able to obtain covariates that have previously been identified as risk factors for COVID-19 in the Swedish population.25 Finally, the study cohort was based on the total population of Sweden, increasing the external validity of the findings to other countries with similar population structures. In summary, our results suggest a substantial waning of vaccine protection against SARS-CoV-2 infection of any severity across all subgroups, but with variations related to vaccine types and schedules. By contrast, protection against severe COVID-19 was better maintained for up to 9 months of follow-up, although some waning became evident after more than 4 months. These findings might have implications for vaccination strategies and public health by strengthening the evidence-based rationale for administration of a third vaccine dose as a booster, where the priority should be specific populations who are at higher risk of severe consequences of COVID-19 due to weaker and more rapidly waning vaccine-elicited immunogenicity.

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