AstraZeneca vaccine provided least protection against infection, showed most rapid decline
Photo: iStock The Institute for Health Metrics and Evaluation (IHME) reworked its COVID-19 model to illustrate how vaccine immunity fell with emergence of new variants, even as the Centre is reportedly discussing the need for a fourth booster dose in the backdrop of cases rising across the world.
The rationality of such a step is questionable for two reasons: Uptake of the first booster dose in India remains at an abysmal 28 per cent of the eligible population one year on, and loyalty to homologous boosters — primary and precautionary doses of the same vaccine — will not yield as much protection as using protein-based vaccines instead.
The rise of the need for a booster dose was propelled by the concept of waning immunity. Protection from COVID-19 infection derived from vaccination reduces rapidly over time, while that against severe disease and death fades at a slower rate.
Read more: Can booster vaccine doses stave off new variants in India?
However, the medical world still doesn’t fully understand the waning of immunity. IHME, an independent population health research centre at the University of Washington Medicine, United States, ranalysed existing literature to fill this lacuna.
IHME compiled “20 studies from nine countries that estimated vaccine efficacy as a function of time since the second dose”. Noting that data for Pfizer-BioNTech, Moderna and AstraZeneca vaccines was limited, the findings revealed the AstraZeneca shot (same formulation as Covishield) not only provided the least protection against infection but also the most rapid decline.
The Johnson & Johnson shot was a close second in a sharp decline in immunity while Modern performed the best. The decline in protection against severe disease saw a similar trend, with the AstraZeneca shot trailing far behind vaccines developed by Pfizer, Moderna and Johnson & Johnson.
Waning immunity against infection
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Source: IHME
Waning immunity against severe disease
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Source: IHME
The new IHME model also sheds light on vaccine effectiveness at preventing infection and severe disease, vaccine-wise and variant-wise. The ability of the vaccine to prevent infection is defined as “stopping transmission of the virus from one person to another. An exposed person will not contract the virus and by definition, they will also not develop symptoms or disease.”
While the effectiveness in preventing severe disease is described as “preventing an exposed person from developing serious symptoms that often require hospitalisation and lead to death”.
The assessment looks at 12 vaccines, including the ones manufactured by AstraZeneca, Pfizer, Moderna, Johnson & Johnson and Sinopharm as well as Covaxin, CoronaVac, CanSino, Novavax and Sputnik-V.
The effectiveness of these doses are measured against COVID-19 variants of concern since the beginning of the pandemic, including the ancestral strain, alpha, beta, gamma, delta and omicron sub-lineages BA.1 / BA.2 and BA.5. “We have reviewed peer-reviewed publications, reports, preprints, medRxiv and news articles since June 2021, and these are continually being searched, in any language,” IHME noted.
The AstraZeneca shot — most widely administered in India — was 94 per cent effective in preventing infection against all COVID-19 variants until omicron, against which its effectiveness reduced considerably to 71 per cent.
For both omicron sub-lineages BA.1 / BA.2 and BA.5, protection against severe disease declined to 36 per cent from 69 per cent against all variants (except omicron).
Covaxin, India’s indigenous vaccine, was 78 per cent effective in preventing an infection from the ancestral strain and Alpha variant, which declined to 76 per cent against beta, gamma and delta, and further to 57 per cent against BA.1 / BA.2 and BA.5.
A similar trend was recorded for Covaxin’s effectiveness against severe diseases — 73 per cent against the ancestral strain and alpha variant, 72 per cent against beta, gamma and delta and 38 per cent against BA.1 / BA.2 and BA.5.
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Novavax — a protein-based vaccine which has been remanufactured by the Serum Institute of India and is still awaiting the drug regulatory body’s approval for roll-out — performed better than other vaccines available in India.
Experts believe it is India’s best option for a booster dose since it will selectively boost just the spike response. Somebody who got two doses of Covishield (by AstraZeneca) will not benefit very much from a third dose of Covishield, owing to the nature of the vaccine, Down To Earth had reported in December 2021.
Two primary doses of Novavax were 89 per cent effective in preventing an infection against the ancestral strain and alpha, 86 per cent against beta, gamma and delta and 65 per cent against the omicron sub-lineages.
Against severe disease, it triggered 83 per cent effectiveness against the ancestral strain and alpha, 82 per cent against beta, gamma and delta and 43 per cent against omicron’s descendents.
The Pfizer shot’s effectiveness in preventing infection dropped from 95 per cent against all COVID-19 variants until omicron to 72 per cent against BA.1 / BA.2 and BA.5.
Against severe disease, its effectiveness dropped from 86 per cent against the ancestral strain to 44 per cent against omicron. Moderna performed similarly. The model does not include the effectiveness of variant-specific vaccines as yet.
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