Chitra Roy, University of Calcutta
As we are getting closer to the COVID-19 vaccine, it is exciting to think of the day when the virus will be gone. But, the COVID-19 vaccine will not be the magic bullet considering its efficacy and the challenges currently being faced in these efficacy studies. Efficacy of a vaccine is the percentage of reduction of a disease in a vaccinated group of people when compared to the unvaccinated group in the most favourable and controlled condition. Presently, a huge number of vaccines are being tested worldwide and some are already in Phase 3 for efficacy assessment. So, there will not be a single vaccine winner! Therefore, standardized approaches for determining and comparing vaccine candidates can lead to the deployment of the most effective vaccine.
For SARS-CoV-2, assessing the vaccine efficacy is quite complex because of our understanding of the pathogen is still evolving. Moreover, ensuring a reliable assessment of the outcome is difficult to promise because multiple variables are determined upon natural exposure of a vaccine to SARS-CoV-2 for clinical trials. For example, older vaccinee participants usually have reduced exposure to SARS-CoV-2 due to their more avoidance of social gatherings and public transport usage since the pandemic, whereas, medical health workers are not only more likely to get exposed to SARS-CoV-2 but also have higher chances to receive huge viral doses. So, these behavioural variables are complex and difficult to control while conducting vaccine efficacy studies.
Therefore, designing a promising SARS-CoV-2 vaccine efficacy trial should be adequately powered for meeting efficacy endpoints. COVID-19 severity and mortality rate varies according to age and sex, which is a crucial determinant for consideration. If Phase 3 vaccine efficacy trial consists of a group of participants of age 20-29years, the mortality rate is expected to be low in this group. However, if a similar study in a group aged above 80years or with comorbidities is performed, the mortality rates would increase in this group and the mortality efficacy endpoint will not be met.
So, is it even possible to conduct better vaccine efficacy study? Currently, it is believed that Human-First or Controlled Human Infection Model (CHIM) trials can provide valuable insights for SARS-CoV-2. CHIM trials are vital for vaccine research and development which aims to carefully select and screen adult individuals as vaccinee or volunteers for better understanding of a disease-transmission, prevention, treatment and have a wider goal to tackle pandemic, endemic and emerging infectious diseases.
Development of COVID-19 CHIM Model can be reliable enough to deal with infection disparity in terms of age and sex. This model includes careful selection of young volunteers who are at a lower risk of developing severe disease symptoms and exposure to known low doses of SARS-CoV-2 for vaccine efficacy assessment. This CHIM trial can also provide some immunological insights. For example, re-exposure of individuals who have already recovered from the infection to SARS-CoV-2 again can help in the identification of a surrogate protection marker. However, CHIM studies require controlled delivery of the pathogenic strain to the volunteer, with good manufacturing practices and meticulous care to prevent any risk for community spread of the pathogenic strain. So, these studies are logistically difficult to practice and cost per volunteer is also high. However, this trial has a substantial value in making a step for ensuring correct vaccine candidate deployment via meaningful epidemiological studies.
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Reference: https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(20)30773-8/fulltext
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