Thread by @covid19indiaorg

On breakthrough infections, vaccines and gap between doses:

A correspondence published in NEJM on 2nd June, details a study on breakthrough infections with a large cohort of 7000+ healthcare workers in a tertiary hospital in India.

nejm.org/doi/full/10.10…

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7000+ received the first dose and 3600+ of them subsequently got the 2nd dose.
Vaccine administered was Covishield.
An additional 5000+ were not vaccinated.

Study period: Jan-Apr 2021.

📌 506 HCWs tested +ve during the study, of which 64% were unvaccinated.

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📌 Of the 7000+ HCWs that got the 1st dose, only 2.6% of them tested positive.
📌 3600+ HCWs got both doses and 2% of them tested positive.
📌 Breakthrough infection, measured as those testing positive 14 days after both doses stood at 1.6%.

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These results point to two important conclusions:

a. Vaccines do provide significant protection against symptomatic infection.
b. While vaccines protect, there can be cases of infection even after 2 doses.

This brings us to the question of variants and vaccine efficacy.

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A key paper from the UK exploring the effectiveness of AstraZeneca (Covishield) and Pfizer vaccines, on variants of concerns, showed significant drop in efficacy for a single dose against the Delta (B.1.617.2) variant.

medrxiv.org/content/10.110…

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Efficacy after 2 doses saw a dip to 60% for Covishield with the B.1.617.2 variant.

📢 Key summary:

✅ B.1.617.2 variant lowers the vaccine efficacy, yet provides significant protection compared to not taking the vaccine.
✅ Efficacy for a single dose dropped to 33%.

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✅ Getting the 2nd dose is critical to provide higher protection with the Delta variant.
✅ A recent genome sequencing study by AIIMS-IGIB on 63 cases of breakthrough infections showed B.1.617.2 to be the dominant variant among the breakthrough cases.
osf.io/fgd4x/

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✅ A key observation from all these studies is that those who got both doses of the vaccine did not experience severe covid or death.

📌 Key implications:

✅ If single dose of vaccine has a lower efficacy against the new variant, should the gap between doses be reduced?

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Excerpt from a recent paper in The Lancet:

"These data therefore suggest that the benefits of delaying the second dose, in terms of wider population coverage, must now be weighed against decreased efficacy in the short-term, in the context of the spread of B.1.617.2."

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✅ Excerpt from same article highlighting the need to speed up the 2nd dose.

"Worldwide, our data highlight the ongoing need to increase vaccine supply to allow all countries to extend second-dose protection as quickly as possible."

thelancet.com/journals/lance…

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✅ We had recently increased the gap between 2 doses of Covishield to 12 - 16 weeks. While trial data suggested an improved efficacy with a delay of 90 days, emerging evidence points to the importance of the 2nd dose against the new dominant variant.

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✅ Emerging data points to the risk of a delayed 2nd dose in the context of the Delta variant. A balance will have to be struck between providing improved protection with 2 doses and covering more persons with 1 dose. In all of this, vaccine availability is the key.

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✅ Lastly, this highlights the importance of genetic sequencing to identify variants of concern and using that data to guide our pandemic response.

Our response to the 2nd wave might have been different if the nature of the B.1.617 variants were acknowledged earlier.

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