Delta variant: What kind of immunity offers the highest protection?

• Many questions persist about immunity against SARS-CoV-2, especially the Delta variant.

• Researchers in Israel have compared the natural immunity acquired from infection and vaccination-acquired immunity.

• Natural immunity plus one dose of vaccine may be the most effective combination to prevent reinfection.

Questions about the long-term protection provided by COVID-19 vaccines abound. How long does the protection last? Are boosters needed? Are the vaccines effective against the Delta variant? Are the vaccines as effective as natural immunity?

A newly released study provides data that may answer some of these questions. The paper has yet to be peer reviewed, and it appears on the preprint server medRxiv.

Long-term protection from vaccination

The study was conducted in Israel, a country with one of the highest rates of COVID-19 vaccination in the world. Because Israel has experienced a recent surge in cases involving the Delta variant, people have expressed many concerns about how effective these vaccines are. In addition, there have been official reports of decreased protection against the virus.

Dr. Elitza S. Theel, director of the Infectious Diseases Serology Laboratory, at the Mayo Clinic, in Rochester, MN, recently told Medical News Today, “From multiple studies, we know that seropositive (antibody-positive) individuals, whether from prior natural infection or vaccination, are at a significantly lower risk of COVID-19, as compared to uninfected or unvaccinated individuals.”

However, the long-term effectiveness of the vaccines against different variants is still unknown, despite occasional reports of waning immunity.

 

Meanwhile, the degree and duration of the protection provided by a past SARS-CoV-2 infection remains unclear. And identifying reinfections, as opposed to prolonged viral shedding, remains a challenge.

Now, sufficient time has passed since the beginning of the pandemic and the deployment of vaccines to finally assess long-term protection as it relates to natural and vaccine-induced immunity.

Comparison of data

Researchers in Israel compared the rates of infection after vaccination — called breakthrough infection — with the rates of reinfection. The researchers obtained the data from the Maccabi Healthcare Services database, Israel’s second-largest health maintenance organization.

Maccabi Healthcare Services is a state-mandated, not-for-profit health fund that covers 26% of Israel’s population and provides a representative sample of Israeli residents, the study authors say. The database includes extensive demographic data, clinical measurements, outpatient and hospital diagnoses, and comprehensive laboratory data.

The study population included people who were at least 16 years old and fell into one of three categories:

• those who had been vaccinated at least 6 months earlier

• those who had a documented SARS-CoV-2 infection at least 6 months earlier

• those who had a SARS-CoV-2 infection at least 6 months earlier and had received one dose of the Pfizer-BioNTech vaccine at least 7 days before the study period began

Natural immunity and one vaccination may offer best protection

The analysis indicated that people who had never had the infection and received a vaccine in January or February of 2021 were up to 13 times more likely to contract the virus than people who had already had the infection.

The researchers also compared reinfection rates among people who had once had a confirmed SARS-CoV-2 infection and were still unvaccinated and people who had once had the infection and had also received one dose of the Pfizer-BioNTech vaccine.

Results showed that the unvaccinated group was twice as likely to contract the infection again, compared with those who had received one dose of the vaccine.

The findings appear to indicate increased protection from natural immunity over vaccine-conferred immunity. This may be due to a more extensive immune response to the SARS-CoV-2 proteins, compared with the immune activation provided by the vaccine, the researchers suggest.

Infection disease experts warn that the findings should not be viewed as an excuse to forego vaccination. In an interview with MNT, Dr. William Schaffner, a professor of infectious diseases at Vanderbilt University Medical Center, in Nashville, TN, said that the vaccines are doing exactly what they were designed to do.

“All the vaccines we have, Moderna, Pfizer, and [Johnson & Johnson], are very effective in preventing hospitalization — a severity of illness that requires admission to the hospital.”

Limitations of the study

The researchers behind the new study acknowledge several limitations. Since the Delta variant was dominant during the study period, the findings concerning natural immunity may not apply to infections with other variants of SARS-CoV-2.

Also, the analysis only included the Pfizer-BioNTech mRNA BNT162b2 vaccine, so the results may not apply to people with other types of COVID-19 vaccine.

Other limitations include a possible underestimation of asymptomatic infections, as these cases are often not tested or recorded.

Finally, the analysis did not account for factors such as social distancing and mask wearing, and this may have affected the results.

Final conclusions

The findings suggest that natural immunity provides longer-lasting and stronger protection against infection, symptomatic disease, and hospitalization due to the Delta variant, compared with the protection of the Pfizer-BioNTech two-dose vaccine, the researchers conclude.

In addition, those with natural immunity exhibit additional protection against the Delta variant when given a single dose of the vaccine, according to the results.

The researchers are continuing to investigate the long-term protection provided by a third dose, or booster, of this vaccine.

Written by Leigh Ann Green on September 6, 2021 — Fact checked by Ferdinand Lali, Ph.D.

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