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After re-analyzing data from 500,000 people in Israel who received the Pfizer vaccine, the researchers suggest that a single dose provides sufficient immunity after approximately 21 days, supporting the UK’s decision to delay the second dose to 12 weeks. .
The coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has raged around the world for more than a year. Although countries have implemented various prevention strategies, cases continue to increase in many parts of the world. However, with the approval of many vaccines, the pandemic is expected to gradually control itself.
In order to prevent the increasing number of cases, in the UK the Joint Committee on Vaccination and Immunization recommended that the second dose of a vaccine be given 12 weeks after the first instead of a few days later, as was done in the phase. III essays. The idea behind this was that giving twice as many people at least one dose in a short time would help reduce the number of cases and possibly reduce the severity of the disease.
There is evidence to show that increasing the gap between the Oxford AstraZeneca vaccine would not have a detrimental effect on the protection offered, but there is no data yet for the Pfizer vaccine.
However, a recent report looked at 500,000 people in Israel who received the Pfizer vaccine from the day after the first dose to day 24. They compared the number of infections in these people between days 13 and 24 with those they had between days 1 and 12 and determined that the efficacy of the vaccine was only 51%. But they also found that infections began to subside only after day 18. However, the study did not determine effectiveness during this latter period. This would have given a better estimate of the efficacy of the vaccine if the second dose was administered after 12 weeks.
Single dose of vaccine can still provide immunity
So a team of researchers from the University of East Anglia, UK, has re-analyzed this data to determine how effective a single dose of Pfizer vaccine would be in a real-world situation. An article published in medRxiv * the preprint server reports its results.
Using data from the previous study, the team analyzed the effectiveness of the vaccine between days 13 and 24 and modeled daily effectiveness using Monte Carlo simulations.
The results of the model showed a total number of 3,077 cases during the 24 days, close to 3,098 reported cases. They found that the vaccine had no apparent effect until day 14, but after that, the effectiveness reached 91% until day 21 and then stabilized.
Therefore, the effect of the vaccine in this population group increased gradually from day 14, peaking at about 90% on day 21. This suggests that the vaccine is very effective after a single dose, but only after about three weeks.
However, there was an increase in the number of cases in the first week after vaccination. This may be because people were less careful after receiving the dose. If so, the true effectiveness of the vaccine may be higher even after the first dose.
Support for delaying the second dose of the vaccine
Regarding the analysis presented in the previous study, the authors note that since they included data from days the vaccine was ineffective, the analysis does not provide information on effectiveness if the second dose was delayed. Furthermore, their analysis could have included mild cases as well, while most Phase III vaccine trials indicate that they are effective in reducing severe rather than mild disease. Therefore, the actual effectiveness in preventing serious illness and death may be better than that calculated here.
Although it is unknown how long protection will last after the first three weeks, studies indicate that antibody levels against natural SARS-CoV-2 infection decrease over time, but are still stable for six months. Therefore, protection is unlikely to decrease until the second booster dose at 12 weeks.
Therefore, this analysis indicates strong protection by the Pfizer vaccine from 21 days after the first dose and supports the UK policy to delay the second dose of all vaccines.
*Important news
medRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered conclusive, guide clinical practice / health-related behavior, or be treated as established information.
