(NEXSTAR) — If a person claims to be “immunized” against COVID-19, what exactly do they mean?

In some cases, it depends on who is making the claim — and which definition they subscribe to.

The terms “immunization” and “vaccination” are often used interchangeably to describe the process by which an individual is made resistant to disease, such as COVID-19. But as far as the Centers for Disease Control and Prevention (CDC) is concerned, “immunization” only occurs after vaccination — and does not refer to protection that comes from a previous infection.

The CDC’s definitions for immunization reads as follows:

• Immunization: A process by which a person becomes protected against a disease through vaccination. This term is often used interchangeably with vaccination or inoculation.

The World Health Organization’s definition differs slightly:

• Immunization is the process whereby a person is made immune or resistant to an infection, typically by the administration of a vaccine.

While WHO’s definition leaves the door open to the potential usage of “immunization” by those who developed immunity from a previous infection, further language on WHO’s website appears to strongly link vaccination with immunization. The latter is even discussed in terms of the “millions of lives” that vaccines are responsible for saving each year, as explained on the organization’s “Vaccines and Immunization” webpage.

But the term “immunization” — and what it means, exactly — has become the subject of recent reports concerning Green Bay Packers quarterback Aaron Rodgers, who tested positive for COVID-19 this week. In August, Rodgers claimed to be “immunized” when asked if he was vaccinated against COVID-19.

Packers coach Matt LaFleur has so far declined to confirm whether Rodgers was indeed vaccinated or merely claiming to have immunity, but multiple reports — from the NFL Network, ESPN and NBC Sports affiliate Pro Football Talk — have cited sources who claim Rodgers was not vaccinated for COVID-19.

In that case, Rodgers’ remark about being “immunized” would, at the very least, be misleading by the CDC’s definition of immunization. It would also be a stretch by WHO’s definition.

If indeed Rodgers was not vaccinated, it’s unclear whether he felt he was protected by a previous infection, or some other homeopathic means. ESPN reported on Wednesday that Rodgers had petitioned the NFL to consider him exempt from being categorized as unvaccinated due to an alternate course of treatment he allegedly underwent, according to sources for the outlet. (Under the NFL’s current policy, unvaccinated players are subject to much stricter protocol than vaccinated players, including separate travel and dining arrangements, daily testing requirements and different rules for being indoors with other teammates, among other protocol.)

Rodgers will miss this Sunday’s game against the Kansas City Chiefs as a result of his positive test. The NFL is also reviewing the incident with the Packers.

Immune responses from a previous infection do not provide the same protections as vaccines. WHO indicates that an individual’s immune response from a previous COVID-19 infection is “not completely understood,” but may last between six and eight months. This response also varies by age and severity of symptoms.

A recent study from the CDC has also found that vaccines offer much stronger immunity to COVID-19 than natural immunity from a previous infection. Specifically, the CDC report found that unvaccinated adults with previous COVID infection were 5.49 times more likely to be re-infected than fully vaccinated individuals with no previously documented infection.

The CDC’s study found these vaccination benefits “trended higher” for those over 65, when compared to vaccinated individuals between 18 and 64 years of age. But unvaccinated persons who were previously infected — across all age groups — were still at higher risk of reinfection.

Both the CDC and WHO urge eligible recipients to get vaccinated with one of the approved COVID-19 vaccines, with WHO calling vaccination “critical” to end the global pandemic.

“Take whatever vaccine is made available to you first, even if you have already had COVID-19,” reads advice from WHO. “It is important to be vaccinated as soon as possible once it’s your turn and not wait.”