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Australian researchers believe they now know how to tell if you are protected from SARS-CoV-2.

A vaccinated person's early immune response can be used to predict the level of protection they will have against the virus over time, the researchers say. 

Mathematicians, clinicians, and scientists at the University of New South Wales' Kirby Institute, the Peter Doherty Institute for Infection and Immunity, and the University of Sydney looked at antibodies to determine the correlate of protection (or correlate of immunity).

"While we have known for some time that neutralising antibodies are likely to be a critical part of our immune response to COVID-19, we haven't known how much antibody you need for immunity," the Kirby Institute's Deborah Cromer said in UNSW's release

"Our work is the strongest evidence to date to show that specific antibody levels translate to high levels of protection from disease."

Predicting protection: Correlates of protection is a bit of a jargon-y way to describe a theoretically simple idea: looking at the type of immune system response you can say means that a person is protected from a pathogen.

Many times, the correlate comes from antibodies; if a person has x level of antibody titres in their blood, they are most likely protected against pathogen y. Other times, the immune response of other cells, like T cells, may be a better indicator; sometimes, we don't have a true correlate of protection at all, "but only useful surrogates, for an unknown protective response," as Stanley Plotkin, inventor of the rubella vaccine, writes.

In their study, published in Nature Medicine, the researchers looked at data from seven different COVID-19 vaccines, examining how the body's early response to the jab correlated to the person being protected from the virus. 

They then determined a specific immune response's relationship to protection, arriving at a number that was accurate enough to predict the efficacy of a new vaccine. 

Speeding up development: Finding an accurate correlate of protection can shorten the amount of time needed to test new vaccines — or boosters — the researchers say.

"Antibody immune levels are much easier to measure than directly measuring vaccine efficacy over time," Cromer said. "So, by measuring antibody levels across the range of new vaccine candidates during early phases of clinical trials, we can better determine whether a vaccine should be used to prevent COVID-19."

In other words, they can check people's antibody response to predict if a vaccine will work, rather than waiting for months to see if they get infected by the virus.

"Our work is the strongest evidence to date to show that specific antibody levels translate to high levels of protection from disease."

Deborah Cromer

This should hold true for variants as well. 

"An added advantage of our work is that (it) allows us to predict how protective an immune response will be against different variants," the University of Sydney's Jamie Triccas said in the release. 

"This work can facilitate decision making by providing the necessary data much earlier on in the vaccine development pipeline and in a far more efficient way."

As the authors note, however, the study does come with a major caveat: their analysis takes the data we have and projects how immunity may change and our response to various variants — and as anyone who's studied it knows, the immune system is hellishly complicated.

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