The changing goalposts of vaccine effectiveness – Dr. Ameet Revankar
The measured efficacy/ effectiveness of the vaccine is only a guide for approval and not a benchmark for claiming superiority of one vaccine over another.
Unknowns are a norm in the uncertain COVID 19 pandemic world. When vaccines were first announced for mass distribution, their efficacy and effectiveness were widely debated. Efficacy refers to the ability of the vaccine to prevent symptomatic disease in the trial participants, whereas effectiveness refers to the same in the real-world setting. One amongst the forerunners was the viral vector-based Oxford-University -AstraZeneca vaccine known by various names in different regions - Covishield, Vaxzevria. The efficacy of this vaccine was reported to be ranging from 50 to 80 percent as per trial data under different dosages/ intervals.
When the mRNA-based vaccines (Pfizer BioNtech & Moderna) vaccines were approved for use in the United States, they showed more than 95 percent efficacy. The real-world effectiveness of these mRNA-based vaccines also showed an effectiveness above 90 percent in preventing symptomatic disease1. As vaccine recipients, we are all desiring vaccines that have higher effectiveness.
However, the vaccine's effectiveness in preventing 'breakthrough' infections also depends on external factors such as the kind of population being vaccinated and their exposure levels. The apparent effectiveness of the vaccines is an outcome heavily dependant on the levels of exposure to the virus. It is virtually impossible to assess the true efficacy of the vaccines unless challenge trials are designed wherein the trial participants are deliberately exposed to the virus, which understandably is unethical. Furthermore, it is pertinent to note that the more vaccine coverage in the community, the lesser exposure would translate into better effectiveness and vice versa. Thus, the measured efficacy/ effectiveness of the vaccine is only a guide for approval and not a benchmark for claiming superiority of one vaccine over another.
The recent paper from the UK confirms this fact wherein both the AstraZeneca and Pfizer vaccines were found to be equally effective in preventing symptomatic disease. Akin to the preventive mitigation measures with Covid 19 that emphasized community rather than individual behavior, the vaccine effectiveness also increases with greater community coverage. With only a minority immunized, the vaccines would fail to protect individuals when the disease incidence or test positivity rate is high during an outbreak, which is currently in India. Several reports of immunized individuals developing symptomatic disease, with questions being asked on the effectiveness of vaccines being used in this program (Covishield predominantly and Covaxin in the minority).
I am pretty confident that the mRNA-based vaccines would also face a similar fate under the present circumstances. Therefore widespread immunization of the community is required not only to rein in the outbreak at a community level but also to ensure that the immunized remain protected. Get vaccinated and motivate others to do the same - Together, we can win!
CDC says fewer than 6,000 Americans have contracted Covid after being fully vaccinated
- Pritchard E, Matthews P, Stoesser N et. Al. Impact of vaccination on SARS-COV-2 cases in the community: a population-based study using the UK’s COVID-19 Infection survey. Available at https://www.medrxiv.org/content/10.1101/2021.04.22.21255913v1. Accessed 24/04/2021
- Associate Professor, Orthodontics, SDM College of Dental Sciences, Dharwad, India
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