Study suggests prior Covid-19 infection more effective against Delta than Pfizer vaccine

New research suggests previously contracting Covid-19 may offer better immunity against Delta than having a Pfizer shot.

Emerging research suggests the Pfizer vaccine may be less effective against the Delta Covid-19 variant than natural immunity developed from a previous infection.

A new study, published in preprint on medRxiv this week, suggests people who have previously contracted Covid-19 were less likely to contract the Delta variant than those who have never been infected but have been vaccinated with the Pfizer vaccine.

The study was conducted in Israel, which has one of the highest Covid-19 vaccination rates in the world.

Researchers examined the medical records of three main groups — people who have never been infected with Covid and have received two shots of the Pfizer vaccine, previously infected individuals who have not been vaccinated, and previously infected individuals who have received a single dose of the vaccine.

A nurse holds a vial of the Pfizer-BioNTech Covid-19 coronavirus vaccine. Picture: Juan Barreto/AFP

These groups were then evaluated for infection, symptomatic disease and Covid-19 related hospitalisation or death during the June 1 to August 14 period of this year when the Delta variant was dominant in Israel.

The study found those who have received two shots of the Pfizer vaccine were 13 times more likely to have a breakthrough infection of the Delta variant than those who had been previously infected with Covid-19 between January and February 2021.

“This study demonstrated that natural immunity confers longer lasting and stronger protection against infection, symptomatic disease and hospitalisation caused by the Delta variant of SARS-CoV-2, compared to the BNT162b2 two-dose vaccine-induced immunity,” the researchers said.

However, the study also found evidence of waning natural immunity.

When comparing to people who had contracted Covid-19 between March 2020 and February 2021, those vaccinated with Pfizer had a six-fold increased risk of breakthrough infection and a seven-fold increased risk for symptomatic disease.

Vaccinated individuals were also found to be at a greater risk of Covid-19 related hospitalisations compared to those who were previously infected.

However, the research also found that individuals who were both previously infected Covid and given a single dose of the vaccine gained additional protection against the Delta variant.

“The long-term protection provided by a third dose, recently administered in Israel, is still unknown,” the researchers noted.

The study, which is yet to be peer reviewed, is the largest real-world analysis comparing natural immunity from previous infection to the protection offered by a Covid-19 vaccine.

The research has already prompted discussion among infectious disease experts and other researchers around the effectiveness of natural immunity.

Swedish physician and immunology researcher at Danderyd Hospital and the Karolinska Institute, Charlotte Thålin, said it was a “textbook example of how natural immunity is really better than vaccination”.

“To my knowledge, it’s the first time [this] has really been shown in the context of Covid-19,” she told Science Magazine.

However, experts have also been quick to emphasise that deliberate infection from Covid-19 to gain natural immunity would put unvaccinated people at severe risk of severe disease, long-lasting symptoms and even death.

“What we don’t want people to say is, ‘All right, I should go out and get infected, I should have an infection party,’” Rockefeller University immunologist Michel Nussenzweig told the publication.

“Because somebody could die.”

Pfizer and other Covid-19 vaccines still remain highly effective against severe disease and death.

University of Washington immunologist Marion Pepper said while the study shows the benefits of natural immunity, it “doesn’t take into account what this virus does to the body to get to that point”.

The retrospective analysis of Covid-19 patients and vaccinated groups is also another limitation of the research, with experts pointing out a study which regularly tests and tracks all participants from the start would yield more accurate results.