Study Design Identification
This is a Randomized Clinical Trial (RCT), specifically a placebo-controlled, double-blind randomized trial (Answer D).
Key Defining Features of This Study Design
The study described contains all the hallmark characteristics of a randomized clinical trial 1:
Random allocation: The 10,000 participants were randomly divided into two groups, which is the fundamental feature distinguishing RCTs from observational studies 1
Prospective intervention: Participants are assigned to receive either the COVID-19 vaccine or placebo, with the intervention occurring after enrollment 2
Placebo control group: One group receives the active intervention (vaccine) while the other receives placebo, which is the gold standard design for minimizing bias 1
Experimental manipulation: The investigators control which participants receive which intervention, rather than observing naturally occurring exposures 2
Why This is NOT the Other Study Types
Not a Cohort Study (A): Cohort studies are observational designs where investigators follow groups with different exposures over time without assigning the exposure 2. In this scenario, the investigators are actively assigning the vaccine vs placebo, not merely observing who chooses to get vaccinated.
Not a Case-Control Study (B): Case-control studies work backwards, starting with people who have the outcome (cases) and those who don't (controls), then looking retrospectively at their exposures 2. This study starts with healthy participants and follows them forward.
Not a Cross-sectional Study (C): Cross-sectional studies measure exposures and outcomes at a single point in time without follow-up 2. This study involves longitudinal follow-up after vaccination to assess outcomes.
Clinical Context of COVID-19 Vaccine Trials
The major COVID-19 vaccine trials followed exactly this design 1:
The Pfizer/BioNTech BNT162b2 trial enrolled 44,000 participants randomized 1:1 to vaccine vs placebo 1
The Moderna mRNA-1273 trial enrolled 30,420 participants randomized 1:1 to vaccine vs placebo 1, 3
Both were double-blind, placebo-controlled RCTs that demonstrated vaccine efficacy of 94-95% 3, 4
Methodological Importance
Placebo-controlled RCTs represent the highest quality evidence for establishing causality and treatment efficacy 1. This design minimizes multiple sources of bias including ascertainment bias and performance bias 1. Studies have shown that unblinded trials can overestimate treatment benefits by approximately 9% compared to placebo-controlled trials 1.