Sterilizing immunity
Sterilizing immunity would be a best-case scenario. It describes an immune system that is armed against a foe, able to fend it off before infection can take hold.
Diseases that we think of as “one-and-done” infections induce such a robust and durable immune response in a single encounter that we cannot be reinfected. In general terms, measles fits into this category, although there are rare reports of people contracting measles more than once.
The bad news is that viruses that infect via the mucus membranes of the nose and throat, like SARS-2, typically don’t induce sterilizing immunity.
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Functional immunity
Functional immunity, on the other hand, may be within reach. In fact, it’s the scenario Menachery sees as most likely.
Under this scenario, people whose immune systems have been primed to recognize and fight the virus — whether through infection or vaccination — could contract it again in the future. But these infections would be cut short as the immune system’s defenses kick into gear. People infected might not develop symptoms or might have a mild, cold-like infection.
“I’m a believer that if you’ve gotten Covid-19, then your likelihood of dying from a second Covid-19 case is very low, if you maintain immunity,” Menachery said.
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Waning immunity
Waning infection, the third scenario, is a variation of functional immunity. In this scenario, people who have been infected or vaccinated would lose their protection over time. But even if immunity wanes, reinfections would be less severe, Menachery said.
“You will never get as sick as you were the first time,” he said.
The man from Hong Kong may be an example of this phenomenon, though details of his immune responses to his first and second infections are not yet public.
This is the pattern seen with the four coronaviruses that cause about 15% of what we consider common colds — OC43, 229E, NL63, and HKU1. People can be reinfected with these viruses after a relatively short period of time.
Thirty years ago, British scientists reported that a year after deliberately infecting a small number of volunteers with 229E, two-thirds became reinfected when again exposed to the virus. “However, the period of virus shedding was shorter than before and none developed a cold,” they wrote.
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Lost immunity
Lost immunity describes a scenario in which people who have been infected would lose all their immune munitions against the virus within some time frame. A reinfection after that point would be like a first infection — carrying all the same risk of severe disease now seen with Covid-19.
None of the experts who spoke to STAT felt this was a possibility.
“I can’t imagine this being a situation where I get infected and then in 10 years, I get infected again and I have zero immunity,” Perlman said.