Recovered Covid patients may need only one mRNA vaccine shot: Scientists

People who have recovered from the novel coronavirus infection may need only one shot as opposed to the recommended two jabs if they are taking the Moderna or Pfizer COVID-19 vaccines, according to a study which suggests ways to minimise doses when supplies are limited.

The yet-to-be peer-reviewed study, posted in the preprint repository medRxiv, assessed the antibody responses in 109 individuals with and without documented pre-existing immunity to the novel coronavirus.

According to the researchers, including Florian Krammer from the Icahn School of Medicine in the US, a single dose of mRNA vaccine elicits very rapid immune responses in individuals already possessing antibodies against the coronavirus from previous exposure to it.

"For individuals with pre-existing immunity to SARS-CoV-2 the first vaccine dose likely immunologically resembles the booster dose in naive individuals," the scientists wrote in the study.

In the research, the scientists analysed mRNA vaccines which use segments of the viral genetic material to enable human cells to make the coronavirus spike proteins.

These proteins train the immune system of vaccine recipients to fight the actual infectious coronavirus when their body encounters the pathogen.

The scientists said the post-vaccine antibody levels in recovered COVID-19 patients are comparable to, or exceed levels, found in those without prior exposure to the virus who received two vaccinations.

Another yet-to-be peer-reviewed study in medRxiv also studied antibody responses to a single dose of the Pfizer-BioNTech or Moderna vaccines in healthcare workers who had previously recovered from the infection.

It found that their antibody levels started peaking at seven days since immunisation, and achieved higher titers and neutralisation in 14 days compared to volunteers exposed to the vaccine for the first time.

"Although we did not have peak titers for these individuals after natural infection, the titers developed after single vaccination was higher than peak titers in inpatients and outpatients with COVID-19, similar to what has been described in primary vaccination after two doses of the spike-based mRNA vaccines," the researchers wrote in the study.

According to the scientists from the University of Maryland School of Medicine in the US, the secondary response occurs through activation of the immune system's memory B cells.

Based on the findings, the researchers recommend a strategy of single dose vaccination for patients who have already had laboratory-confirmed COVID-19.

They said those who have recovered from the disease can be placed lower on the vaccination priority list.

Commenting on the two studies, Eleanor Riley, Professor of Immunology and Infectious Disease at the University of Edinburgh in the UK, said the findings are "very reassuring", adding that the vaccines are "very effectively boosting the immunity induced by infection."

While the two studies suggest that people who have had laboratory-confirmed COVID-19 infection may only require one dose of the vaccine, Riley, who was unrelated to the studies, said incorporating this into a mass vaccination programme may be logistically complex.

He believes it may be safer to ensure that everyone gets two doses.

The preprint platfrom, medRxiv, also cautions that the posted reports are preliminary in nature and have not been certified by peer review.

"They should not be relied on to guide clinical practice or health-related behavior," it adds.

Lawrence Young, Virologist and Professor of Molecular Oncology, University of Warwick in the UK, believes this question can be resolved with further studies.

"...we should be doing further studies which look at giving previously infected individuals one dose of an mRNA vaccine," said Young, who was also unrelated to the two studies.

"If future work can confirm this high level of immunity post a single mRNA vaccine in this group of individuals, this could become a viable option when there are concerns around vaccine supply," he added in a statement.