Part 6/14:
The global development landscape has faced challenges in acquiring sufficient evidence of long-term immunity, requiring ongoing studies and booster assessments. Moreover, scientists analyze immune correlates—specific antibodies or T-cell responses—that can serve as markers of protection, potentially simplifying future vaccine approvals and bypassing lengthy phase 3 trials. Human challenge trials, where volunteers are intentionally exposed to the virus, emerge as a controversial yet promising method to accelerate efficacy assessments, especially useful as natural infection rates decline.