CDC: Prior COVID-19 Infection Protected Against Delta

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Verywell Health / Shideh Ghandeharizadeh 

Key Takeaways

  • Prior infection to COVID-19 provides some protection against Delta variant.
  • Getting vaccinated and having a prior infection provided the highest defense against Delta.
  • Health experts say getting a COVID-19 booster shot can provide some immunity against Omicron and potential new variants.

A new Centers for Disease Control and Prevention (CDC) study shows that people who'd previously been infected with COVID-19 (unvaccinated and vaccinated) were better protected against the Delta variant compared to those who were just vaccinated alone—suggesting natural immunity was more effective against that particular variant.

Additionally, researchers found that people who were both vaccinated and survived a previous infection from COVID-19 had the highest protection against Delta during the surge last year. Protection was the lowest among people who had never been infected or vaccinated.

“People with the strongest immunity are people who have what is called, 'hybrid immunity' which is being vaccinated and then infected,” Erica Pan, MD, California’s state epidemiologist, told Verywell. “Every time you get exposed either to the vaccine or infection, it can improve your immunity.” 

From the entire study period—May to November 2021—people who had been vaccinated and had prior infection obtained immunity, lowering their rates of hospitalization compared to those in unvaccinated groups.

“These results suggest that vaccination protects against COVID-19 and related hospitalization and that surviving previous infection protects against a reinfection,” the authors of the study said in a press release

Natural Immunity Offered Some Protection

Researchers analyzed data from about 1.1 million cases in California and New York between the end of May and November 2021. They studied and analyzed the risk of COVID-19 and hospitalization among four groups of people:

  • Vaccinated with prior infection
  • Vaccinated without prior infection
  • Unvaccinated with prior infection
  • Unvaccinated without prior infection

In the initial findings of the study, those with prior infection had higher case rates compared to those who were vaccinated and had no history of prior infection. But months later, as the Delta variant became more dominant across the US, those initial findings changed. According to the study, once Delta was circulating, people who survived a previous infection had lower case rates than those who were vaccinated alone. 

“Before the Delta variant, COVID-19 vaccination resulted in better protection against a subsequent infection than surviving a previous infection," Benjamin Silk, PhD, lead for the CDC’s surveillance and analytics on the Epi-Task Force, said in a call with media. "When looking at the summer and the fall of 2021, when Delta became dominant in this country, however, surviving a previous infection now provides greater protection against subsequent infection than vaccination.”

Health experts stress that despite prior infection providing some level of protection and immunity, getting vaccinated remains the safest strategy against COVID-19. 

“In our data from California and New York State, absolutely people that are infected compared to people who have never been infected or never been vaccinated do have some protection and are less likely to be hospitalized or die, but vaccines are still the safest way to protect yourself,” Pan said.

This isn't the first time researchers have found that natural immunity can offer protection alongside vaccines. Several studies throughout 2021 have mirrored these results.

When Should You Get Your Shot?

The CDC recommends receiving your shots immediately after recovering from illness, unless you've been treated with monoclonal antibodies. If you have received antibody treatments, you should wait 90 days. Some studies suggest that antibody levels reach higher counts if you wait 90 days post-infection to get vaccinated. The World Health Organization (WHO) states that people can consider delaying vaccination for six months after infection. But they add that you should ask a healthcare provider for advice.

Researchers note that the results of this CDC study do not apply to the Omicron variant of the virus, which accounts for 95% of cases in the United States and does not factor in the effect booster doses may have.

Does That Mean It’s Better to Naturally Get Infected With COVID-19? 

According to experts like Pan, it is not better to purposefully get infected with COVID-19. She states since COVID-19 itself is an infection, when someone gets infected, it can cause serious outcomes including hospitalizations and death. 

The CDC study supports this, stating that acquiring immunity through natural infection carries significant risks. Specifically, initial infection among unvaccinated people increased their risk for serious illness, hospitalization, long-term effects, and death.

Robert G. Lahita, MD, PhD, director of the Institute for Autoimmune and Rheumatic Disease at Saint Joseph Healthcare, told Verywell choosing natural immunity and not having any protection—like the vaccine—puts someone in danger of becoming seriously ill. 

“If you get COVID and are unvaccinated, you have no way of knowing whether you will become critically ill," Lahita said. "If you get Omicron—which is noticeably milder than other variants—that does not protect you from Delta, which is the most severe variant we have seen on average. The bottom line is you are taking a huge risk by not being vaccinated and relying on natural immunity.” 

Pan added that getting infected with COVID-19 can lead to long-term effects and symptoms. 

“We’re learning more and more about long COVID and how many people get long-term symptoms," Pan said. "There’s a range of studies going on that show long COVID can cause difficulty breathing, fatigue, headache, muscle pain, and even reports of neurological problems—like people not being able to think straight. Which is why vaccination is the best way to protect people.”

How Long Does Natural Immunity Last Anyways?

According to Pan, the length of immunity depends on the individual and other factors including what variant they were infected with, how sick the individual was during a previous infection, and age. 

“How we get immunity and how long it lasts are still under intense studies, but we don’t have clear straightforward answers to that yet,” Pan said.

Other experts support this and say natural immunity can last up to three months or even longer. Some studies find that immunity can even last from five to 12 months. But it depends on the individual.

“We don’t know how long natural immunity will last or can last because everyone is different," Lahita said. "One person may get COVID and have immunity for that variant for three months, while another person may have immunity for six months or a year. Antibody tests can tell you whether you have neutralizing antibodies, which will last for some time, but there is absolutely no way to say for sure how long natural immunity lasts for each individual.” 

What This Means For You

Vaccination is still the safest measure against the virus and potential new variants. You can find an appointment near you here.

What About Omicron and New Variants?

The CDC stated that this data could not apply to the current surge caused by the Omicron variant since the variant behaves differently than Delta. In addition, the study took place before most people had received a booster dose.

Despite these factors, researchers say as new variants emerge, vaccination remains the safest strategy for preventing COVID-19 infection, serious illness, hospitalizations, and death. Additionally, primary vaccination and booster shots are recommended for all eligible people. 

“Even with the highly infectious Omicron variant, getting a booster provides a lot of additional protection against infections, hospitalizations, and death,” Pan said. “Really our message from this data is still that vaccination is the safest way to get protection against COVID-19 and even if you’ve been infected, that vaccine still provides additional protection.” 

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Bozio CH, Grannis SJ, Naleway AL, et al. Laboratory-confirmed COVID-19 among adults hospitalized with COVID-19–like illness with infection-induced or mRNA vaccine-induced SARS-CoV-2 immunity — nine states, January–September 2021. MMWR Morb Mortal Wkly Rep 2021;70:1539–1544. doi:10.15585/mmwr.mm7044e1

  3. Zhong D, Xiao S, Debes A et al. Durability of Antibody Levels After Vaccination With mRNA SARS-CoV-2 Vaccine in Individuals With or Without Prior Infection. JAMA. 2021;326(24):2524. doi:10.1001/jama.2021.19996

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  5. Centers for Disease Control and Prevention. COVID data tracker.

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By Alyssa Hui
Alyssa Hui is a St. Louis-based health and science news writer. She was the 2020 recipient of the Midwest Broadcast Journalists Association Jack Shelley Award.