“Those more likely to get vaccine seek care more, thus more likely to be cases
- Table of Content -
Vaccinated persons less likely to seek care/testing due to COVID-19-like illness due to perception of protection”
– WHO, The World Health Organization
• Excerpts :
Study designs :
Table 3. Potential biases of COVID-19 vaccine effectiveness studies
•
• Care seeking behaviour/ access to care :
Those more likely to get vaccine seek care more, thus more likely to be cases
Large
• Care seeking based on vaccine status :
Vaccinated persons less likely to seek care/testing due to COVID-19-like illness due to perception of protection
Small-moderate
Increase in CaCo and cohort Decrease in TND, if vaccine confers some protection
• Non-specific vaccine effect :
Vaccine prevents diseases for which controls seek care
Small (has not been shown)
• Prior infection :
If known prior SARS-CoV-2 infection, less likely to get vaccinated
Small-moderate (depends on seroprevalence/ past incidence of infection)
• Spurious waning :
Unvaccinated individuals become immune through natural infection faster than vaccinated (46)
Small soon after vaccine campaign, large with increasing time since campaign
• Survivorship :
Unvaccinated more likely to die of COVID-19 : Small
Health care seeking/access: People who have better access or higher tendency to utilize health care will both be more likely to get vaccinated and present for care when symptomatic, including with COVID-19. In traditional case-control studies, this would lead to over-representation of vaccinated individuals as cases, which would decrease the VE estimate. TNDs partially mitigate this bias since all enrolled persons have sought care. However, TNDs can lead to collider bias, whereby health seeking and SARS-CoV-2 infection both lead to testing, which is usually thought to be of lower magnitude than health care seeking bias (64).
Diagnostic bias: Clinicians might be less likely to order COVID-19 testing in vaccinated patients, reasoning that vaccinated patients are protected against COVID-19. TND partially addresses this bias since all participants are tested. The decision to test potential study subjects should not be based on clinicians’ decisions but on prespecified protocol-defined criteria. These criteria should then be applied to all (or to a random sample of ) eligible patients regardless of clinical testing decisions.
Misclassification of the outcome: Outcome misclassification occurs due to imperfect laboratory test performance in diagnosing COVID-19 infection (72). Erroneous test results can be both false negative and false positive. Because both rRT-PCR and rapid antigen tests tend to have higher specificity than sensitivity, false-negative test results are more common. However, false-positive tests can result in greater bias in estimating the VE (63, 73). Misclassification can bias the VE more in TND than in cohort or traditional case-control studies because the control group in TND will be over-represented with false- negative cases compared with the source population. Despite these concerns, misclassification bias is likely small when using tests with high analytic sensitivity and specificity (see Section 8. Laboratory ( )).
https://apps.who.int/iris/rest/bitstreams/1337417/retrieve
