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.

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.