After reanalyzing data from 500,000 people in Israel who received the Pfizer vaccine, researchers suggest a single dose provides sufficient immunity after about 21 days, thus providing support for the UK’s decision to delay the second dose to 12 weeks.
The coronavirus disease 2019 (COVID-19) pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has spread rampantly across the globe for more than a year. Although countries have implemented several prevention strategies, cases are still surging in many parts of the world. With the approval of many vaccines, however, it is hoped that the pandemic will gradually be brought under control.
With a view to preventing the increasing number of cases, in the UK, the Joint Committee on Vaccination and Immunization made a recommendation that the second dose of a vaccine be given 12 weeks after the first rather than after a few days as done in the Phase III trials. The idea behind this was that giving twice the people at least one dose in a short time would help bring down the number of cases and possibly reduce disease severity.
There is evidence showing 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 were given 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 to those between days 1 and 12 and determined the vaccine efficacy was only 51%. But, they also found that infections started to decrease only after day 18. However, the study did not determine effectiveness during this latter period. This would have given a better estimate of how effective the vaccine would be if the second dose was given after 12 weeks.
Single vaccine dose may still provide immunity
So, a team of researchers from the University of East Anglia, UK, have reanalyzed this data to determine how effective a single dose of the Pfizer vaccine would be in a real-world situation. A paper published in the medRxiv* preprint server reports their results.
Using data from the previous study, the team analyzed vaccine effectiveness between days 13 and 24 and modeled daily effectiveness using Monte Carlo simulations.
The model results showed a total number of 3,077 cases over the 24 days, close to the 3,098 cases reported. They found the vaccine had no apparent effect until day 14, but after that, the effectiveness reached 91% until day 21 and then leveled off.
Thus, the effect of the vaccine in this set of population increased gradually from day 14, peaking at about 90% at day 21. This suggests the vaccine is highly 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 vaccine second dose
Regarding the analysis presented in the previous study, the authors note that since they included data from days when the vaccine was ineffective, the analysis does not provide information about effectiveness if the second dose was delayed. Furthermore, their analysis could have included mild cases also, whereas most Phase III vaccine trials indicate they are effective in reducing severe disease rather than mild disease. So, actual effectiveness in preventing severe disease 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 antibody levels to natural SARS-CoV-2 infection decline over time but are still stable over six months. Thus, it’s unlikely that protection will decline until the second booster shot at 12 weeks.
Thus, this analysis indicates strong protection by the Pfizer vaccine starting 21 days after the first dose and provides support for the UK’s policy of delaying the second dose of all vaccines.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.