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A physician for Novak Djokovic attested that because the Serbian tennis star had caught and recovered from COVID-19, he was eligible to apply for a medical exemption to not get the COVID-19 vaccine. 

Catching and recovering from the virus offers some natural protection against further infection — but natural immunity differs from person to person and wanes over time. As tens of thousands of Australians become infected daily, the question remains: should there be different rules for the recovered? 

What level of natural immunity is there?

When it first emerged, there was considerable speculation about whether COVID could be caught twice. Reinfections were initially rare — by August 2020 just a handful of people who had caught the virus became reinfected. 

An initial study from November 2020 found that just 0.7% of those in the United States who had recovered caught the virus again. 

But hopes were short-lived. As natural immunity waned and the virus evolved and mutated and variants emerged, scores of people caught the same or new strains, causing surge after surge of case numbers. One study found that reinfection occurred for most people 16 months after recovery. 

Walter and Eliza Hall Institute infectious disease professor Marc Pellegrini says natural immunity is a result of antibodies produced by the immune system to fight the virus. But how many antibodies are produced and how long they stay in a person’s system varies from person to person. 

“It’s very hard to know whether someone who’s been infected — particularly those that become quite unwell — have actually got a robust immune response,” he said. 

“Speculatively, people that get very sick have defined themselves as probably not having a particularly robust immune response against the virus, whereas people who are asymptomatic or recover quickly are more likely to have had a robust immune response.”

Relying on natural immunity is risky

There’s a reason variants have emerged in areas with low vaccination rates: the virus is more likely to mutate in a person with a poor immune response to it. There’s speculation the Omicron variant mutated during a chronic infection in an immunocompromised person, such as an untreated HIV/AIDS patient.

Vaccines use the body’s natural immune response to produce antibodies, but produce a much higher antibody response than the immune system would naturally produce. This gives the virus less opportunity to mutate and change. 

“The virus evolves very fast in people that have had a poor immune response because it can escape [the immune system] more readily, giving it an opportunity to change its colours a little bit,” Pellegrini said.  

Natural immunity is no match for vaccines

It’s difficult to draw conclusions on natural immunity as antibody production differs from person to person. It’s much easier to draw conclusions on vaccines because while the immune response is still different for everyone, the baseline level of antibody production is much higher. 

Pfizer and Moderna are developing a vaccine specific to Omicron; Pfizer announced this morning its vaccine would be ready by March.

Although the Omicron-specific vaccine isn’t as important here — with boosters rolled out to more than 3.2 million Australians — Pellegrini says it will be key in countries with low vaccination rates. And although a booster dose of an mRNA vaccine will offer high levels of protection from severe disease from Omicron and other strains, rolling out a variant-specific vaccine will offer greater protection for those receiving their first or second vaccine dose. 

“​​Giving the best vaccine for the variants that are most common at the time, rather than having to give the more suboptimal vaccine they’re having to give more often, is the best course of action,” Pellegrini said.