Pfizer SARS-CoV-2 vaccine effective from day 11 post 1st vaccination

New research from a team of scientists in Australia suggests that the second ‘booster’ dose of the Pfizer-BioNTech vaccine may not increase vaccine efficacy against severe acute respiratory syndrome (SARS-CoV-2), the causative agent of coronavirus disease 2019 (COVID-19).

The results suggest one dose of the vaccine could be sufficient in providing a robust immune response 11 days after inoculation.

The findings could help shape policy as vaccine rollout has been slow in countries such as the United States and Canada. This could also increase with vaccine distribution efforts in developing countries where supplies have been scarce.

This opens up possibilities where rapid short-term protection that could be used for ‘ring fencing’ vaccination following an outbreak or cluster. Also, it could be given to travelers in the fortnight prior to their planned trip. Given there was no demonstrated immediate improvement in giving a second vaccination after 3 or 4 weeks it also raises the potential to provide a single dose (at least in the short term when supplies are limited) and provide health services with more flexibility in administering vaccines and in the timing of a second dose if found to be warranted.”

The study “Reanalysis of the Pfizer mRNA BNT162b2 SARS-CoV-2 vaccine data fails to find any increased efficacy following the boost: Implications for vaccination policy and our understanding of the mode of action” is available as a preprint on the medRxiv* server, while the article undergoes peer review.

How they did it

The research team reexamined Pfizer-BioNTech phase III trial data from day 1 to 111 in the placebo and experimental groups. They also looked at data from Moderna’s vaccine trial. However, since the numbers of COVID-19 cases in the Moderna trial were low in the first few weeks, they did not have sufficient data to assess it. Instead, Moderna’s trial data was used for comparison purposes with Pfizer’s trial data.

They studied the efficacy of vaccination from day 11 to day 28 and compared the efficacy of the second vaccine dose delivered on day 28 up to day 111.

Pfizer vaccine shows complete efficacy 11 days after vaccination

Results showed the Pfizer-BioNTech mRNA vaccine provided a robust protective immune response 11 days after the first vaccination dose. They also found that the second vaccination did not help to increase efficacy.

Specifically, the first vaccine dose helped develop detectable neutralizing antibodies before the second dose.

While Moderna’s sample size was limited, compared with the Pfizer-BioNTech’s mRNA vaccine, there continued to be no significant difference in vaccine efficacy between days 11 to 35 and 7 days after the second vaccine was administered.

Results raise more questions about the vaccine

Previous studies have found that the Pfizer-BioNTech vaccine increased immunoglobulin binding to the S1 domain on days 21 and day 28 after vaccination. Given that the study found a lack of an effect in the second vaccine dose, it raises questions about the two vaccines’ underlying protective mechanism.

For example, mRNA vaccines can be potent inducers of innate immunity. SARS-CoV-2 specific secreted IgA, has been proposed as important following natural infection. Antibody effector functions may also contribute to protection. A better understanding of potential multifactorial protection mechanisms other than serum antibody binding or serum neutralizing antibody may be important for optimizing vaccine schedules, assessing the potential impact of new antigenic variants and developing new generation anti-COVID-19 vaccines,” write the researchers.

The team’s reanalysis also looked at long-term immunity up to day 111, but only after the second dose was given. If a single vaccine can elicit full immunity, the researchers question how long the vaccine immunity will last.

Potentially having prolonged immunity from one shot would help vaccine rollout and help people who have trouble finding or scheduling a second vaccination on a specific day.

Given the number of global cases reaching over 114.7 million and the number of global deaths surpassing 2.5 million, a one-dose vaccine could help widen access to vaccine-induced immunity against SARS-CoV-2.

*Important Notice

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.

Written by

Jocelyn Solis-Moreira

Jocelyn Solis-Moreira graduated with a Bachelor's in Integrative Neuroscience, where she then pursued graduate research looking at the long-term effects of adolescent binge drinking on the brain's neurochemistry in adulthood.