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1.
EuropePMC; 2022.
Preprint in English | EuropePMC | ID: ppcovidwho-337754

ABSTRACT

Introduction: Individuals with a prior severe acute respiratory corona virus 2 (SARS-CoV-2) infection have a moderate to high degree of protection against reinfection, though seemingly less so when the Omicron variant of SARS-CoV-2 started to circulate. The aim of this study was to evaluate the vaccine effectiveness (VE) against SARS-CoV-2 reinfection, that is, in individuals with prior SARS-CoV-2 infection, during periods with different dominant SARS-CoV-2 variants. Methods A nationwide cohort study design including all individuals with a confirmed SARS-CoV-2 infection, who were alive and residing in Denmark between 1 January 2020 and 31 January 2022 were used. Using Danish nationwide registries, we obtained information on SARS-CoV-2 infections, Coronavirus Disease 2019 (COVID-19) vaccination, age, sex, comorbidity, staying at hospital and region of affiliation. The study population included were individuals with prior SARS-CoV-2 infection. Crude and adjusted estimates of VE against SARS-CoV-2 reinfection with 95% confidence intervals (CIs) were calculated using Poisson and Cox regression models, respectively. The VE estimates were calculated separately for three periods with different dominant SARS-CoV-2 variants (Alpha (B.1.1.7), Delta (B.1.617.2), or Omicron (B.1.1.529)) and by time since vaccination using unvaccinated as the reference. Findings The study population comprised of 209,814 individuals infected before or during the Alpha period, 292,978 before or during the Delta period and 245,530 before or during the Omicron period. Of these, 40,281 individuals had completed their primary vaccination series during the Alpha period (19.2%), 190,026 during the Delta period (64.9%) and 158,563 during the Omicron period (64.6%). VE against reinfection following any COVID-19 vaccine type administered in Denmark, peaked at 85% (95% CI: 37% to 97%) at 104 days or more after vaccination during the Alpha period, 88% (95% CI: 81% to 92%) 14-43 days after vaccination during the Delta period and 60% (95% CI: 58% to 62%) 14-43 days after vaccination during the Omicron period. Waning immunity was observed, and was most pronounced during the Omicron period. Interpretation This study shows that, in previously infected individuals, completing a primary vaccination series was associated with a significant protection against SARS-CoV-2 reinfection compared with no vaccination for all three variant periods. Even though vaccination seems to protect to a lesser degree against reinfection with the Omicron variant, these findings are of public health relevance as they show that previously infected individuals still benefit from COVID-19 vaccination in all three variant periods.

2.
EuropePMC; 2022.
Preprint in English | EuropePMC | ID: ppcovidwho-335783

ABSTRACT

Background The continued occurrence of more contagious SARS-CoV-2 variants and waning immunity over time require ongoing re-evaluation of the vaccine effectiveness (VE). This study aimed to estimate the effectiveness in two age groups (12-59 and 60 years or above) of two and three vaccine doses (BNT162b2 mRNA or mRNA-1273 vaccine) by time since vaccination against SARS-CoV-2 infection and COVID-19-related hospitalization in an Alpha, Delta and Omicron dominated period. Methods A Danish nationwide cohort study design was used to estimate VE against SARS-CoV-2 infection and COVID-19-related hospitalization with the Alpha, Delta and Omicron variants. Information was obtained from nationwide registries and linked using a unique personal identification number. The study included all residents in Denmark aged 12 years or above (18 years or above for the analysis of three doses) in the Alpha (February 20 to June 15, 2021), Delta (July 4 to November 20, 2021) and Omicron (December 21, 2021 to January 31, 2022) dominated periods. VE estimates including 95% confidence intervals (CIs) were calculated using Cox proportional hazard regression models with adjustments for age, sex and geographical region. Vaccination status was included as a time-varying exposure. Findings In the oldest age group, VE against infection after two doses was 91.0% (95% CI: 88.5;92.9) for the Alpha variant, 82.2% (95% CI: 75.3;87.1) for the Delta variant and 39.9% (95% CI: 26.4;50.9) for the Omicron variant 14-30 days since vaccination. The VE waned over time and was 71.5% (95% CI: 54.7;82.8), 49.8% (95% CI: 46.5;52.8) and 4.7% (95% CI: 0.2;8.9) >120 days since vaccination against the three variants, respectively. Higher estimates were observed after the third dose with VE estimates against infection of 86.0% (Delta, 95% CI: 83.3;88.3) and 57.6% (Omicron, 95% CI: 55.8;59.4) 14-30 days since vaccination. Among both age groups, VE against COVID-19-related hospitalization 14-30 days since vaccination with two or three doses was 94.8% or above for the Alpha and Delta variants, whereas among the youngest age group, VE estimates against the Omicron variant after two and three doses were 62.4% (95% CI: 46.3;73.6) and 89.8% (95% CI: 87.9;91.3), respectively. Conclusions Two vaccine doses provided high protection against SARS-CoV-2 infection and COVID-19-related hospitalization with the Alpha and Delta variants with protection waning over time. Two vaccine doses provided only limited protection against SARS-CoV-2 infection and COVID-19-related hospitalization with the Omicron variant. The third vaccine dose substantially increased the protection against Delta and Omicron.

3.
PLoS Med ; 18(12): e1003874, 2021 12.
Article in English | MEDLINE | ID: covidwho-1581902

ABSTRACT

BACKGROUND: The recommendations in several countries to stop using the ChAdOx1 vaccine has led to vaccine programs combining different Coronavirus Disease 2019 (COVID-19) vaccine types, which necessitates knowledge on vaccine effectiveness (VE) of heterologous vaccine schedules. The aim of this Danish nationwide population-based cohort study was therefore to estimate the VE against Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection and COVID-19-related hospitalization and death following the first dose of the ChAdOx1 vaccine and the combination of the ChAdOx1/mRNA vaccines. METHODS AND FINDINGS: All individuals alive in or immigrating to Denmark from 9 February 2021 to 23 June 2021 were identified in the Danish Civil Registration System. Information on exposure, outcomes, and covariates was obtained from Danish national registries. Poisson and Cox regression models were used to calculate crude and adjusted VE, respectively, along with 95% confidence intervals (CIs) against SARS-CoV-2 infection and COVID-19-related hospitalization or death comparing vaccinated versus unvaccinated individuals. The VE estimates were adjusted for calendar time as underlying time and for sex, age, comorbidity, country of origin, and hospital admission. The analyses included 5,542,079 individuals (97.6% of the total Danish population). A total of 144,360 individuals were vaccinated with the ChAdOx1 vaccine as the first dose, and of these, 136,551 individuals received an mRNA vaccine as the second dose. A total of 1,691,464 person-years and 83,034 SARS-CoV-2 infections were included. The individuals vaccinated with the first dose of the ChAdOx1 vaccine dose had a median age of 45 years. The study population was characterized by an equal distribution of males and females; 6.7% and 9.2% originated from high-income and other countries, respectively. The VE against SARS-CoV-2 infection when combining the ChAdOx1 and an mRNA vaccine was 88% (95% CI: 83; 92) 14 days after the second dose and onwards. There were no COVID-19-related hospitalizations or deaths among the individuals vaccinated with the combined vaccine schedule during the study period. Study limitations including unmeasured confounders such as risk behavior and increasing overall vaccine coverage in the general population creating herd immunity are important to take into consideration when interpreting the results. CONCLUSIONS: In this study, we observed a large reduction in the risk of SARS-CoV-2 infection when combining the ChAdOx1 and an mRNA vaccine, compared with unvaccinated individuals.


Subject(s)
/administration & dosage , COVID-19/immunology , COVID-19/prevention & control , SARS-CoV-2 , Adult , Comorbidity , Denmark , Female , Hospitalization , Humans , Male , Middle Aged , Risk , Treatment Outcome , Vaccination
4.
2021.
Preprint in English | Other preprints | ID: ppcovidwho-295541

ABSTRACT

Background At the end of 2020, Denmark launched an immunization program against SARS-CoV-2. The Danish health authorities prioritized persons currently living in long-term care facilities (LTCF residents) and frontline healthcare workers (HCW) as the first receivers of vaccination. Here we present preliminary population based vaccine effectiveness (VE) estimates in these two target groups. Methods The study was designed as a retrospective registry- and population-based observational cohort study including all LTCF residents and all HWC. The outcome was a polymerase chain reaction confirmed SARS-CoV-2, and VE was estimated for different periods following first and second dose. We used Poisson and Cox regressions to estimate respectively crude and calendar time-adjusted VE for the BNT162b2 mRNA Covid-19 Vaccine from Pfizer/BioNTech with 95% confidence intervals (CI) for vaccinated versus unvaccinated. Results A total of 39,040 LTCF residents (median age at first dose;84 years, Interquartile range (IQR): 77-90) and 331,039 HCW (median age at first dose;47 years, IQR: 36-57) were included. Among LTCF residents, 95.2% and 86.0% received first and second dose from 27 December 2020 until 18 February 2021, for HWC the proportion was 27.8% and 24.4%. During a median follow-up of 53 days, there were 488 and 5,663 confirmed SARS-CoV-2 cases in the unvaccinated groups, whereas there were 57 and 52 in LTCF residents and HCW within the first 7 days after the second dose and 27 and 10 cases beyond seven days of second dose. No protective effect was observed for LTCF residents after first dose. In HCW, VE was 17% (95% CI;4-28) in the > 14 days after first dose (before second dose). Furthermore, the VE in LTCF residents at day 0-7 of second dose was 52% (95% CI;27-69) and 46% (95% CI;28-59) in HCW. Beyond seven days of second dose, VE increased to 64% (95% CI;14-84) and 90% (95% CI;82-95) in the two groups, respectively. Conclusion The results were promising regarding the VE both within and beyond seven days of second vaccination with the BNT162b2 mRNA Covid-19 Vaccine currently used in many countries to help mitigate the global SARS-CoV-2 pandemic. Impact of the research So far, observational studies of the real-word effectiveness of the mRNA Vaccine BNT162b2 has been limited to the period after the administration of the first dose. This is the first report to date to present vaccine effectiveness (VE) estimates after the second BNT162b2 mRNA Covid-19 Vaccine. We estimated a VE of 52% and 46% in LTCF residents and HCW within seven days, which increased to 64% and 90% in the two groups respectively beyond seven days of immunization. These findings supports maintaining a two-dose schedule of the BNT162b2 mRNA Covid-19 Vaccine.

5.
J Infect Dis ; 225(2): 219-228, 2022 01 18.
Article in English | MEDLINE | ID: covidwho-1522221

ABSTRACT

BACKGROUND: Studies presenting severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection fatality rate (IFR) for healthy individuals are warranted. We estimate IFR by age and comorbidity status using data from a large serosurvey among Danish blood donors and nationwide data on coronavirus disease 2019 (COVID-19) mortality. METHODS: Danish blood donors aged 17-69 years donating blood October 2020-February 2021 were tested with a commercial SARS-CoV-2 total antibody assay. IFR was estimated for weeks 11 to 42, 2020 and week 43, 2020 to week 6, 2021, representing the first 2 waves of COVID-19 epidemic in Denmark. RESULTS: In total, 84944 blood donors were tested for antibodies. The seroprevalence was 2% in October 2020 and 7% in February 2021. Among 3898039 Danish residents aged 17-69 years, 249 deaths were recorded. The IFR was low for people <51 years without comorbidity during the 2 waves (combined IFR=3.36 per 100000 infections). The IFR was below 3‰ for people aged 61-69 years without comorbidity. IFR increased with age and comorbidity but declined from the first to second wave. CONCLUSIONS: In this nationwide study, the IFR was very low among people <51 years without comorbidity.


Subject(s)
Antibodies, Viral/blood , Blood Donors , COVID-19/diagnosis , SARS-CoV-2/isolation & purification , Adolescent , Adult , Aged , COVID-19/blood , COVID-19/epidemiology , Comorbidity , Denmark/epidemiology , Female , Humans , Male , Middle Aged , Seroepidemiologic Studies , Young Adult
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