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How long should you wait to get your second Covishield vaccine if you were infected after receiving the first injection?

This article was published on
May 7, 2021

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The World Health Organization recommends an interval of eight and twelve weeks between the first and second doses of the AstraZeneca vaccine. Further studies are needed to evaluate whether a longer amount of time between the two doses than three months if safe and effective. Waiting longer than four weeks between doses appears to make the vaccine more effective.

The World Health Organization recommends an interval of eight and twelve weeks between the first and second doses of the AstraZeneca vaccine. Further studies are needed to evaluate whether a longer amount of time between the two doses than three months if safe and effective. Waiting longer than four weeks between doses appears to make the vaccine more effective.

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What our experts say

Covishield (also called Vaxzevria) is AstraZeneca's COVID-19 vaccine. In clinical trials researchers studied how effective the vaccine was after one dose. Early results showed that the vaccine didn't produce a strong enough immune responses in trial participants, so researchers then decided to add a second shot, setting time between doses at four weeks. Those findings showed that the vaccine prevented symptomatic infections in 76% of participants 15 days or more after receiving their second shot. For adults over 65 years of age, this number was 85% and the vaccine prevented severe disease and hospitalizations in 100% of all trial participants. However, a pre-print analysis in the Lancet compared 17,000 people — some whose shots were 12 weeks apart, others whose shots were 6 weeks apart. The vaccine was more effective in people whose shots were twelve weeks apart. It appears that delaying the second dosage of the vaccine may strengthen the immune system's response to the virus. This is important because nations like the United Kingdom, India, and Spain have increased the amount of time between the first and second doses of Covishield. These nations are hoping to use their current supplies to get a greater number of people partially vaccinated, rather than a fewer number fully vaccinated. This will hopefully decrease rates of transmissions and hospitalizations so the health system is not consistently overwhelmed with COVID-19 patients. The risk with this strategy is that more "breakout" infections can occur after just a single dose. The World Health Organization recommends an interval of 8-12 weeks between the first and second doses of the vaccine. Further studies are needed to evaluate a longer amount of time between the two doses than three months.

Context and background

AstraZeneca realized its vaccine may be more effective when an error in their clinical trials gave participants a smaller first dose than they were supposed to, followed by a second dose more than four weeks later. This error led to the idea that a longer time frame between the first and second dose might make the vaccine have a higher efficacy. Nations have experimented with delaying the second dosage of vaccine up to 16 weeks. However, currently published research has only evaluated the safety and efficacy of vaccines administered within a 12-week time frame. There is not enough information to determine whether or not a second vaccine dose given after this length of time is as safe or effective. 

Resources

  1. Single-dose administration and the influence of the timing of the booster dose on immunogenicity and efficacy of ChAdOx1 nCoV-19 (AZD1222) vaccine: a pooled analysis of four randomised trials (The Lancet)
  2. Interim recommendations for use of the ChAdOx1-S [recombinant] vaccine against COVID-19 (AstraZeneca COVID-19 vaccine AZD1222, SII Covishield, SK Bioscience) (World Health Organization)
  3. Covid-19: New data on Oxford AstraZeneca vaccine backs 12 week dosing interval (The BMJ)
  4. AZD1222 US Phase III primary analysis confirms safety and efficacy (AstraZeneca)
  5. What scientists do and don’t know about the Oxford–AstraZeneca COVID vaccine (Nature)
  6. Latest results put Oxford–AstraZeneca COVID vaccine back on track (Nature)
  7. AstraZeneca’s COVID-19 vaccine: benefits and risks in context (European Medicines Agency)
  8. Delaying 2nd AstraZeneca COVID shot may boost efficacy (Center for Infectious Disease Research and Policy)
  9. Spain extends AstraZeneca dose gap to 16 weeks, beyond EU approved limit (Reuters)
  10. Did India get its COVID vaccine strategy wrong? (Al Jazeera)
  11. AstraZeneca vaccine: 3-month dosage interval might be preferable (Medical News Today)

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