It's a question that scientists have been trying to answer since the start of the pandemic, one that is central to the rancorous political debates over coronavirus vaccine policies: How much immunity does someone have after recovering from a coronavirus infection, and how does it compare with immunity provided by vaccination?
The Centers for Disease Control and Prevention has weighed in for the first time in a detailed report released with little fanfare Friday evening.
Reviewing scores of research studies and its own unpublished data, the agency found that both infection-induced and vaccine-induced immunity are durable for at least six months -- but that vaccines are more consistent in their protection and offer a huge boost in antibodies for people previously infected.
In comparing the two types of immunity, scientists said research shows vaccination provides a "higher, more robust, and more consistent level of immunity to protect people from COVID-19 than infection alone."
Coronavirus infections can cause severe disease or no symptoms at all, and the CDC found that antibody levels vary widely from one individual to another after an infection. The report also notes that there is no test authorized by the Food and Drug Administration that would enable doctors and the public to reliably measure an individual's protection from disease. And although higher levels of neutralizing antibodies generally signal higher protection, scientists don't know precisely what level of antibodies will protect an individual.
The CDC's bottom line: Given what's known and not known about immunity, people who have been infected with the virus should still get vaccinated.
More than 45 million people in the United States have had confirmed coronavirus infections, and tens of millions more -- the exact number is unknown -- have had undocumented cases.
The science brief echoes another study, also released by the CDC on Friday, that the agency said showed a higher level of protection from vaccines than from previous infection alone.
That study said vaccinated patients hospitalized with covid-like symptoms were less likely to test positive for the virus than those who had recovered from a coronavirus infection months earlier. In other words, the patients vaccinated against the coronavirus were more likely to have some illness other than covid.
Immunity from a prior infection has been a vexing issue for the Biden administration as it continues to push vaccinations as the key to ending the pandemic. Some Republican members of Congress have seized on what they term "natural immunity" to push back against the vaccine mandates favored by the White House.
Sen. Mike Lee, R-Utah, and Rep. Diana Harshbarger, R-Tenn., have put forward legislation to take such immunity into account. Harshbarger's Natural Immunity is Real Act would require federal agencies to "acknowledge, accept, agree to truthfully present, and incorporate, the consideration of natural immunity as it pertains to COVID-19 with respect to the individuals subject to the applicable regulations."
The two lawmakers and co-sponsors of the legislation have argued that the Biden administration must "follow the science" showing that people who have recovered from covid-19 have durable immunity.
"Now more than ever, we need to pursue every scientific measure -- such as natural immunity -- that can help mitigate the pandemic without threatening people's jobs, our economy, or denying Americans access to everyday life activities based on COVID-19 vaccination status," Harshbarger said in a news release last month announcing the bill.
But the act does not offer any suggestion for how agencies would confirm immunity from prior infections or incorporate such immunity into a vaccine policy.
"The politics are as complicated as the science," Andrew Pavia, a professor of pediatrics and infectious diseases at the University of Utah, said in an email Monday. He said of the new CDC science review, "I think there is a tension between conveying the scientific gray areas and the need to combat the 'natural infection is better' misinformation that has taken hold. I think the review threads that needle well."
In the brief, CDC scientists evaluated more than 90 peer-reviewed studies and preprint publications to understand the level of protection in people who have immunity from prior infection and those with immunity from vaccines. For people who have been infected, multiple studies have shown that vaccination provides a boost in the immune response and further reduces the risk of a repeat infection.
"Although there appears to be varying evidence regarding the relative protection that occurs after surviving COVID-19 as compared with completing vaccination, there is substantial immunologic and increasing epidemiologic evidence that vaccination following infection further increases protection against subsequent illness among those who have been previously infected," the CDC science brief said.
To complicate the issue further, some strong supporters of vaccinations have said vaccine policies could contemplate a kind of hybrid immunity, generated by a combination of infection and perhaps just one vaccine dose. In some countries, a person who has recovered from covid-19 is considered fully vaccinated after a single dose.
"I think with more data, we might consider one infection equivalent to one immunizing event that could count as one of the two or three doses that people need of their vaccine," Pavia said in a phone interview.
David Rubin, a pediatrician and director of PolicyLab at Children's Hospital of Philadelphia, said he counsels his patients and family members who have had covid infections that they could get by with just one shot to bolster their immunity.
"Consider that a dose of the vaccine," Rubin said, referring to a coronavirus infection. "If you've had it, get at least one dose of the vaccine."
As of September, France, Germany, Italy and Spain are among more than a dozen countries that recommend that people without underlying health conditions who have already been infected receive one dose of a vaccine if it comes in a two-shot regimen.
But for most countries and the United States, the definition of fully vaccinated does not incorporate previous infection. The White House strategy for ending the pandemic relies heavily on vaccinating as many people as possible, including those who have already had covid-19 or tested positive for the virus.
William Schaffner, an infectious-disease doctor at Vanderbilt University School of Medicine who is a liaison member to the CDC's panel of independent vaccine advisers, said policies have to be simple enough for officials to implement and for people to understand. He invokes the "KISS" rule: "Keep it simple, stupid."
And the simple fact is that vaccines improve antibody levels whether a person has been infected or not, Schaffner said.