On Measuring Vaccine Effectiveness with Observational Study Designs

Real-world effectiveness of nirsevimab immunisation against bronchiolitis in infants: a case-control study in Paris, France

BACKGROUND

Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis in infants. Nirsevimab, an RSV-neutralising monoclonal antibody, was approved for use in the EU in 2022, and a national immunisation campaign began in France in September, 2023. We aimed to assess the effectiveness of nirsevimab in reducing paediatric emergency department visits (and subsequent hospitalisations) for all-cause bronchiolitis and RSV-associated bronchiolitis.

METHODS

In this case-control study in a paediatric emergency department in Paris, France, we included all infants aged 12 months or younger who attended the department between Oct 14, 2023, and Feb 29, 2024, and whose nirsevimab status was known. Infants were classed as cases if they had all-cause bronchiolitis; all other infants were classed as controls. The primary outcome was the effectiveness of nirsevimab against paediatric emergency department visits for all-cause bronchiolitis during the 2023-24 RSV season. Secondary outcomes were paediatric emergency department visits for RSV-associated bronchiolitis; hospitalisations for all-cause bronchiolitis, RSV-associated bronchiolitis, and severe RSV-associated bronchiolitis requiring supplemental oxygen or feeding by nasogastric tube; and severe RSV-associated bronchiolitis requiring admission to the paediatric intensive care unit. Effectiveness estimates were adjusted for age, week of paediatric emergency department visit, and sex.

FINDINGS

Our study included 2786 infants, 864 with all-cause bronchiolitis (cases) and 1922 without bronchiolitis (controls). 178 (21%) of the 864 cases had received nirsevimab, and 305 (35%) cases were hospitalised for all-cause bronchiolitis. 200 (72%) of the 277 cases tested for RSV were positive, of whom 22 (11%) had received nirsevimab. 701 (36%) of 1922 infants in the control group had received nirsevimab. The effectiveness of nirsevimab against paediatric emergency department visits for all-cause bronchiolitis was 47% (95% CI 33-58). Nirsevimab effectiveness was 83% (71-90) against paediatric emergency department visits for RSV-associated bronchiolitis, 59% (42-71) against hospitalisations for all-cause bronchiolitis, 83% (72-90) against hospitalisations for RSV-associated bronchiolitis (91% [78-96] against those necessitating supplement oxygen and 88% [74-95] against those necessitating feeding via a nasogastric tube). Nirsevimab did not significantly reduce admissions to the paediatric intensive care unit (67% [95% CI -100 to 95]).

INTERPRETATION

During the first French national immunisation campaign, a single dose of nirsevimab effectively reduced paediatric emergency department visits (both all-cause visits and visits related to RSV-associated bronchiolitis) and subsequent hospitalisations.

FUNDING

None.

Homologous or heterologous administration of mRNA or adenovirus-vectored vaccines show comparable immunogenicity and effectiveness against the SARS-CoV-2 Omicron variant

BACKGROUND

Heterologous prime-boost schedules have been employed in SARS-CoV-2 vaccination, yet additional data on immunogenicity and effectiveness are still needed.

RESEARCH DESIGN AND METHODS

Here, we measured the immunogenicity and effectiveness in the real-world setting of the mRNA booster dose in 181 subjects who had completed primary vaccination with ChAdOx1, BNT162b2, or mRNA1273 vaccines (IMMUNO_COV study; protocol code 18,869). The spike-specific antibody and B cell responses were analyzed up to 6 months after boosting.

RESULTS

After an initial slower antibody response, the heterologous ChAdOx1/mRNA prime-boost formulation elicited spike-specific IgG titers comparable to homologous approaches, while spike-specific B cells showed a higher percentage of CD21-CD27- atypical cells compared to homologous mRNA vaccination. Mixed combinations of BNT162b2 and mRNA-1273 elicited an immune response comparable with homologous strategies. Non-significant differences in the Relative Risk of infection, calculated over a period of 18 months after boosting, were reported among homologous or heterologous vaccination groups, indicating a comparable relative vaccine effectiveness.

CONCLUSIONS

Our data endorse the heterologous booster vaccination with mRNA as a valuable alternative to homologous schedules. This approach can serve as a solution in instances of formulation shortages and contribute to enhancing vaccine strategies for potential epidemics or pandemics.

Infection with SARS-CoV-2 following Second Dose Pfizer-BioNTech mRNA COVID-19 Vaccine BNT162b2 in Danish Adolescents Aged 12-18 Years: A Real-World Nationwide Danish Cohort Study

In this real-world cohort study based on Danish nationwide registers, the cumulated proportion, relative risk (RR) of SARS-CoV-2 breakthrough infections, and vaccine effectiveness (VE) were investigated in adolescents aged 12-18 years following vaccination with the BNT162b2 vaccine compared to unvaccinated controls. Adolescents with and without vaccination with the first dose of BNT162b2 between 1 May and 30 September 2021 were included. Effect estimates include proportions with a positive SARS-CoV-2 RT-PCR test among vaccinated and unvaccinated, RR, and VE at three different time points. During Delta-dominance, VE was first 97.6% (95% CI 96.3-98.4), then 96.2% (95% CI 95.4-96.9) in the age group 12-15 and 95.1% (95% CI 94.1-96.0) followed by 95.5% (95% CI 94.8-96.1) in the age group 16-18 years, respectively. During Omicron dominance, VE was 5.8% (95% CI 4.6-7.0) in ages 12-15 years and 9.2% (95% CI 7.7-10.6) in ages 16-18 years. Thus, BNT162b2-vaccine protection was limited during the Omicron era.

Measuring vaccine protection when the population is mostly vaccinated

OBJECTIVES

This study aims to address limitations in assessing vaccine protection using the classical vaccine effectiveness (VE) measure, especially in contexts where a significant portion of the population is already vaccinated or infected.

STUDY DESIGN AND SETTING

We propose using the adjusted number of cases (ANC) as a building block for deriving vaccine effectiveness measures. This approach accounts for biases arising from small and unrepresentative unvaccinated reference groups with incomplete data. We demonstrate the use of these measures for assessing the protection conferred by a booster dose against severe COVID-19 using data from Israel.

RESULTS

The use of ANC and the derived measures reveals a more comprehensive understanding of the complex immunity landscape compared to traditional VE measures. This approach enables meaningful comparisons between different vaccination categories and provides insights to inform policy decisions.

CONCLUSION

In situations with widespread vaccination and prior infections, traditional VE measures can be limited in their informative value. Using the ANC offers a more robust and insightful assessment of vaccine effectiveness. A demonstration of the evaluation of booster dose protection against severe COVID-19 in Israel underscores the importance of adopting complementary measures to guide public health strategies.

Correction of vaccine effectiveness derived from test-negative case-control studies

BACKGROUND

Determining the vaccine effectiveness (VE) is an important part of studying every new vaccine. Test-negative case-control (TNCC) studies have recently been used to determine the VE. However, the estimated VE derived from a TNCC design depends on the test sensitivity and specificity. Herein, a method for correction of the value of VE derived from a TNCC study is presented.

METHODS

An analytical method is presented to compute the corrected VE based on the sensitivity and specificity of the diagnostic test utilized. To show the application of the method proposed, a hypothetical TNCC study is presented. In this in silico study, 100 000 individuals referring to a healthcare system for COVID-19-like illness were tested with diagnostic tests with sensitivities of 0.6, 0.8, and 1.0, and specificities ranging from 0.85 to 1.00. A vaccination coverage of 60%, an attack rate of 0.05 for COVID-19 in unvaccinated group, and a true VE of 0.70, were assumed. In this simulation, a COVID-19-like illness with an attack rate of 0.30 could also affect all the studied population regardless of their vaccination status.

RESULTS

The observed VE ranged from 0.11 (computed for a test sensitivity of 0.60 and specificity of 0.85) to 0.71 (computed for a test sensitivity and specificity of 1.0). The mean computed corrected VE derived from the proposed method was 0.71 (the standard deviation of 0.02).

CONCLUSIONS

The observed VE derived from TNCC studies can be corrected easily. An acceptable estimate for VE can be computed regardless of the diagnostic test sensitivity and specificity used in the study.