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What do waning Covid-19 antibodies tell us about immunity and vaccines?

By Kate Kelland
Growing evidence that Covid-19 antibody levels can wane swiftly after someone is infected is not necessarily bad news for immunity, experts said, and does not mean protection offered by coronavirus vaccines will be weak or short-lived.

Specialists in immunology and viruses warned against reading too much into studies of antibody levels in the blood of people previously infected with Covid-19, cautioning that antibody readings do not translate directly into levels of protective immunity.

"The concentration of antibodies in your blood does not equal immunity," said Eleanor Riley, a professor of immunology and infectious disease at Britain's University of Edinburgh.

She and other experts said reports earlier this week which suggested immunity to Covid-19 might decline in line with falling blood antibody levels failed to account for the many complexities in how the body builds immunity to infections.

"Immunity is not something we can just wrap up in measuring an antibody or T-cell response," she told Reuters.

"Immunity is about the system working together so that next time you come across the infection, you either won't get it at all or won't get seriously ill from it. That's protective immunity."

While antibodies induced by natural Covid-19 infection may start to decline in few months, as a study by researchers at Imperial College found, the many potential Covid-19 vaccines in development are designed to induce more durable immunity by invoking strong so-called immune memory.

"Antibody responses are usually short-lived because once they have done their job you don't need them," said Jonathan Stoye, head of virology at Britain's Francis Crick Institute.

"But that doesn't mean that immunity, either induced by infection or by vaccination, is necessarily short-lived: Memory cells can respond to and combat a new infection."

Since SARS-CoV-2, the virus that causes Covid-19, is a new human virus, scientists don't yet know what levels of immunity will turn out to be protective. But many of the vaccine makers are touting both the antibody and T-cell responses, which are increasingly seen as important to lasting immunity.

"The immune system is very complicated. We know antibodies are important, but they're not the whole story," said Lawrence Young, a professor of molecular oncology at Britain's Warwick University. "The important thing here is immune memory."

Key to the process of immunity are memory cells known as T- and B-lymphocytes. Having made antibodies to a certain virus in an initial infection, the body uses these cells to remember that pathogen, "so when you are next exposed to the virus, the antibody response kicks in much sooner", Young said.

With vaccines, a key feature is that scientists developing them can select as targets the most important bits of the pathogen - in Covid-19's case these include the so-called "spike protein" on the surface of SARS-CoV-2 virus - to get the strongest and most lasting memory responses from T and B lymphocytes.

Some vaccines also contain stimulators or boosters, known as adjuvants, which can supercharge the response, and others are designed to be given in multiple doses to ensure higher concentrations of antibodies will create stronger memories.

"The idea is that while the natural infection may give you poor memory that may not last, the vaccine will give you strong memory that does last," said Danny Altmann, a professor of immunology at Imperial College London.

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