Validity of the estimates of oral cholera vaccine effectiveness derived from the test-negative design

Prospective observational studies to provide confidence in the protection conferred by newer generation, inactivated oral cholera vaccines: a proposal

Inactivated oral cholera vaccines (OCVs) are a cornerstone of international efforts to control cholera, and are currently deployed from a global stockpile for the control of epidemics and endemic hotspots, as well as for humanitarian emergencies. One inactivated OCV (with tradenames Shanchol™ and Euvichol-Plus™) is used in the stockpile, but the number of available doses is inadequate to meet the rapidly rising demand for OCVs from countries affected by cholera. Newer, simplified inactivated OCVs under development offer the possibilities of lower expense and higher production yields, and could expand the stockpile. However, their clinical development is made complex because placebo-controlled randomised trials of OCV efficacy are no longer ethically permissible and because the serum vibriocidal antibodies used to measure OCV responses are not correlates of OCV protection against cholera. Here, we propose an observational study design with features to enhance methodological rigor to provide credible evidence of protection against cholera by these newer vaccines.

Protocol on establishing a prospective enhanced surveillance of vaccine preventable diseases in residential aged care facilities in Central Queensland, Australia: an observational study

INTRODUCTION

Infectious diseases are a major cause of mortality and morbidity among the highly vulnerable occupants of residential aged care facilities (RACFs). The burden of vaccine preventable diseases (VPDs) among RACFs residents is mostly unknown and there is a lack of quality data from population-based prospective VPD surveillance in RACFs. The increasing burden of emerging and existing VPDs (eg, COVID-19, influenza, pneumococcal, pertussis and varicella-zoster) necessitates the establishment of an active enhanced surveillance system to provide real-time evidence to devise strategies to reduce the burden of VPDs in RACFs.

METHOD AND ANALYSIS

This study proposes a prospective active enhanced surveillance that will be implemented in RACFs across the Central Queensland (CQ) region. The study aims to measure the burden, identify aetiologies, risk factors, predictors of severe outcomes (eg, hospitalisations, mortality) and impact of the existing National Immunization Program (NIP) funded vaccines in preventing VPDs in this vulnerable population. CQ Public Health Unit (CQPHU) will implement the active surveillance by collecting demographic, clinical, pathological, diagnostic, therapeutic and clinical outcome data from the RACFs based on predefined selection criteria and case report forms as per routine public health practices. Descriptive statistics, univariate and multivariate regression analysis will be conducted to identify the predictors of morbidity and clinical outcomes following infection.

ETHICS AND DISSEMINATION

The study has been approved by the CQHHS Human Research Ethics Committee (HREC) (reference number HREC/2021/QCQ/74305). This study involves data that is routinely collected as part of the surveillance of notifiable conditions under the Public Health Act 2005. The CQHHS HREC approved a request to waive consent requirements of study participants as researchers will be provided non-identifiable data. The findings from the study will be actively disseminated through publication in peer-reviewed journals, conference presentations, social and print media, federal, state, and local authorities to reflect on the results that may facilitate revision of policy and highlight the stakeholders, funding bodies both locally and internationally.

Adjustment for Disease Severity in the Test-Negative Study Design

The test-negative study design is often used to estimate vaccine effectiveness in influenza studies, but it has also been proposed in the context of other infectious diseases, such as cholera, dengue, or Ebola. It was introduced as a variation of the case-control design, in an attempt to reduce confounding bias due to health-care-seeking behavior, and has quickly gained popularity because of its logistic advantages. However, examination of the directed acyclic graphs that describe the test-negative design reveals that without strong assumptions, the estimated odds ratio derived under this sampling mechanism is not collapsible over the selection variable, such that the results obtained for the sampled individuals cannot be generalized to the whole population. In this paper, we show that adjustment for severity of disease can reduce this bias and, under certain assumptions, makes it possible to unbiasedly estimate a causal odds ratio. We support our findings with extensive simulations and discuss them in the context of recently published cholera test-negative studies of the effectiveness of cholera vaccines.

Temporal Confounding in the Test-Negative Design

In the test-negative design, routine testing at health-care facilities is leveraged to estimate the effectiveness of an intervention such as a vaccine. The odds of vaccination for individuals who test positive for a target pathogen is compared with the odds of vaccination for individuals who test negative for that pathogen, adjusting for key confounders. The design is rapidly growing in popularity, but many open questions remain about its properties. In this paper, we examine temporal confounding by generalizing derivations to allow for time-varying vaccine status, including out-of-season controls, and open populations. We confirm that calendar time is an important confounder when vaccine status varies during the study. We demonstrate that, where time is not a confounder, including out-of-season controls can improve precision. We generalize these results to open populations. We use our theoretical findings to interpret 3 recent papers utilizing the test-negative design. Through careful examination of the theoretical properties of this study design, we provide key insights that can directly inform the implementation and analysis of future test-negative studies.

Alternative observational designs to estimate the effectiveness of one dose of oral cholera vaccine in Lusaka, Zambia

We conducted a matched case-control (MCC), test-negative case-control (TNCC) and case-cohort study in 2016 in Lusaka, Zambia, following a mass vaccination campaign. Confirmed cholera cases served as cases in all three study designs. In the TNCC, control-subjects were cases with negative cholera culture and polymerase chain reaction results. Matched controls by age and sex were selected among neighbours of the confirmed cases in the MCC study. For the case-cohort study, we recruited a cohort of randomly selected individuals living in areas considered at-risk of cholera. We recruited 211 suspected cases (66 confirmed cholera cases and 145 non-cholera diarrhoea cases), 1055 matched controls and a cohort of 921. Adjusted vaccine effectiveness of one dose of oral cholera vaccine (OCV) was 88.9% (95% confidence interval (CI) 42.7–97.8) in the MCC study, 80.2% (95% CI: 16.9–95.3) in the TNCC design and 89.4% (95% CI: 64.6–96.9) in the case-cohort study. Three study designs confirmed the short-term effectiveness of single dose OCV. Major healthcare-seeking behaviour bias did not appear to affect our estimates. Most of the protection among vaccinated individuals could be attributed to the direct effect of the vaccine.

The Use of Test-negative Controls to Monitor Vaccine Effectiveness: A Systematic Review of Methodology

BACKGROUND

The test-negative design is an increasingly popular approach for estimating vaccine effectiveness (VE) due to its efficiency. This review aims to examine published test-negative design studies of VE and to explore similarities and differences in methodological choices for different diseases and vaccines.

METHODS

We conducted a systematic search on PubMed, Web of Science, and Medline, for studies reporting the effectiveness of any vaccines using a test-negative design. We screened titles and abstracts and reviewed full texts to identify relevant articles. We created a standardized form for each included article to extract information on the pathogen of interest, vaccine(s) being evaluated, study setting, clinical case definition, choices of cases and controls, and statistical approaches used to estimate VE.

RESULTS

We identified a total of 348 articles, including studies on VE against influenza virus (n = 253), rotavirus (n = 48), pneumococcus (n = 24), and nine other pathogens. Clinical case definitions used to enroll patients were similar by pathogens of interest but the sets of symptoms that defined them varied substantially. Controls could be those testing negative for the pathogen of interest, those testing positive for nonvaccine type of the pathogen of interest, or a subset of those testing positive for alternative pathogens. Most studies controlled for age, calendar time, and comorbidities.

CONCLUSIONS

Our review highlights similarities and differences in the application of the test-negative design that deserve further examination. If vaccination reduces disease severity in breakthrough infections, particular care must be taken in interpreting vaccine effectiveness estimates from test-negative design studies.

Oral cholera vaccine delivery strategy in India: Routine or campaign?-A scoping review

Oral Cholera Vaccine (OCV) has been recognized as an adjunct tool for prevention and control of cholera. However, policy directions are currently unavailable in India to guide the vaccine delivery. We conducted a scoping review to inform the policy about the scopes and challenges of different strategic choices of OCV delivery in India in light of current evidences, highlighting the scope of new research.

METHODS

Adopting the Arksey and O'Malley Framework for review, we searched for literatures on "efficacy", "effectiveness", and "cost" of oral cholera vaccine delivery through different strategies in Pubmed and Scopus.

RESULTS

We found that the protective efficacy of OCV depends on its coverage. Evidence on effectiveness of OCV are available for both reactive and pre-vaccination campaigns. Reactive high-risk vaccination is more effective than reactive ring and mass vaccination. Pre-vaccination campaigns are more effective than reactive vaccination when vaccine availability is adequate. Pre-vaccination through school campaigns in 1-14 years age group have been cost effective in India. Vaccination campaigns in under-5 children are also cost effective in spite of low efficacy due to the scope of averting a higher number of cases. However, no evidence is available regarding efficacy and effectiveness of OCV in children <1 year as well as the effectiveness of delivering OCV through routine immunization.

CONCLUSION

Little evidence exist to depict mass-campaign as more economic and effective than routine expanded programme on immunization (EPI) session for delivery of OCV. Considering operational feasibility, it needs to be explored whether OCV delivery strategy is compatible with India's current EPI, if it can be introduced in routine immunization at measles containing vaccine age-schedule, optionally preceded by a campaign in targeted hot-spots in the 1-14 year age-group. Safety and efficacy data of OCV during infancy as well as hot-spot surveillance are pre-requisites for formulation of such EPI policy.

Effectiveness of oral cholera vaccine in preventing cholera among fishermen in Lake Chilwa, Malawi: A case-control study

BACKGROUND

In response to a cholera outbreak among mobile, difficult-to-reach fishermen on Lake Chilwa, Malawi in 2016, a novel vaccine distribution strategy exploited the proven vaccine thermostability. Fishermen, while taking the first vaccine dose under supervision, received the second dose in a sealed bag, and were told to drink it two weeks later. This study assessed short-term vaccine protection of this strategy.

METHODS

Patients with diarrhoea admitted to health facilities around lake were interviewed and a stool sample collected for PCR testing. Vaccine effectiveness was assessed in a case-control test-negative design by comparing cases (PCR-positive for V. cholerae O1) and controls (patients with diarrhoea but PCR-negative) and with the screening method that compared the proportions of vaccinated among cholera cases versus the general fishermen population.

RESULTS

Of 145 study participants, 120 were fishermen living on the lake. Vaccine effectiveness at three-months was 90.0% [95%CI:38.8;98.4] among fishermen and 83.3% [95%CI: 20.8; 96.5] among all participants in the case-control test-negative design, and 97.5% [95%CI: 90.9;99.3] with the screening method.

CONCLUSION

This strategy was effective in providing short-term protection in fishermen against cholera. Further research is needed to determine the adding value of the second dose and to identify the optimal vaccination strategies for different contexts.

Measurement of Vaccine Direct Effects Under the Test-Negative Design

Test-negative designs are commonplace in assessments of influenza vaccination effectiveness, estimating this value from the exposure odds ratio of vaccination among individuals treated for acute respiratory illness who test positive for influenza virus infection. This approach is widely believed to recover the vaccine direct effect by correcting for differential health-care-seeking behavior among vaccinated and unvaccinated persons. However, the relationship of the measured odds ratio to true vaccine effectiveness is poorly understood. We derived the odds ratio under circumstances of real-world test-negative studies. The odds ratio recovers the vaccine direct effect when 2 conditions are met: 1) Individuals' vaccination decisions are uncorrelated with exposure or susceptibility to the test-positive or test-negative conditions, and 2) vaccination confers "all-or-nothing" protection (whereby certain individuals have no protection while others are perfectly protected). Biased effect-size estimates arise if either condition is unmet. Such bias might suggest misleading associations of vaccine effectiveness with time since vaccination or the force of infection of influenza. The test-negative design could also fail to correct for differential health-care-seeking behavior among vaccinated and unvaccinated persons without stringent criteria for enrollment and testing. Our findings demonstrate a need to reassess how data from test-negative studies can inform policy decisions.

Oral cholera vaccines and their impact on the global burden of disease

With one-third of nations at risk of cholera, we can expect to experience massive, rapidly disseminated, and prolonged cholera outbreaks such as those recently experienced in Yemen and Haiti. The prevention of cholera outbreaks like these includes the provision of potable water, sanitation, and hygiene (WASH). This approach has been known for generations. However, it will be many years before universal global access to WASH is achieved. While working toward universal WASH, study data has shown that licensed and WHO prequalified cholera vaccines are important tools for cholera prevention. Oral inactivated whole-cell vaccines such as Shanchol and Euvichol-plus provide well-documented direct benefits to vaccine recipients and to the unimmunized through herd protection. Manufacturers have now increased the cholera vaccine supply, and since 2013 vaccine doses have been available for emergency and endemic control through a global stockpile. Advances in packaging and vaccine temperature control, reduced vaccine costs, the inclusion of pregnant women in vaccine campaigns, and a targeted approach to high incidence endemic areas are further increasing the usefulness of these vaccines for reducing the global cholera burden.

Comparison of two control groups for estimation of oral cholera vaccine effectiveness using a case-control study design

BACKGROUND

Case-control studies to quantify oral cholera vaccine effectiveness (VE) often rely on neighbors without diarrhea as community controls. Test-negative controls can be easily recruited and may minimize bias due to differential health-seeking behavior and recall. We compared VE estimates derived from community and test-negative controls and conducted bias-indicator analyses to assess potential bias with community controls.

METHODS

From October 2012 through November 2016, patients with acute watery diarrhea were recruited from cholera treatment centers in rural Haiti. Cholera cases had a positive stool culture. Non-cholera diarrhea cases (test-negative controls and non-cholera diarrhea cases for bias-indicator analyses) had a negative culture and rapid test. Up to four community controls were matched to diarrhea cases by age group, time, and neighborhood.

RESULTS

Primary analyses included 181 cholera cases, 157 non-cholera diarrhea cases, 716 VE community controls and 625 bias-indicator community controls. VE for self-reported vaccination with two doses was consistent across the two control groups, with statistically significant VE estimates ranging from 72 to 74%. Sensitivity analyses revealed similar, though somewhat attenuated estimates for self-reported two dose VE. Bias-indicator estimates were consistently less than one, with VE estimates ranging from 19 to 43%, some of which were statistically significant.

CONCLUSIONS

OCV estimates from case-control analyses using community and test-negative controls were similar. While bias-indicator analyses suggested possible over-estimation of VE estimates using community controls, test-negative analyses suggested this bias, if present, was minimal. Test-negative controls can be a valid low-cost and time-efficient alternative to community controls for OCV effectiveness estimation and may be especially relevant in emergency situations.