No One Knows if You Need Another Covid Booster

The US food and the drug agency is urging you to get an annual Covid refresher. The problem is that the data isn’t clear if you need one.

Covid is not going anywhere. SARS-CoV-2 is still circulating at significant levels in the US and many European countries, with Covid emerging as a major ongoing cause of disease. Booster shots can protect against the worst effects, but these are shots in the blue: Critical illness insurance, but may not be necessary. Because we don’t know how long their protection against serious illnesses actually lasts.

It’s time we found out, but that means changing focus. At the level of basic biology, this means paying less attention to the antibodies that make vaccines and focusing more on another very important but overlooked part of the immune system: memory T cells. “You can tell who needs booster shots by how long the memory cells last,” says Paul Offit, professor of vaccinology at the University of Pennsylvania and FDA vaccine advisor.

The immune system is complex, but basically it consists of three parts. There’s an innate immunity, the physical or chemical barriers – like your skin or the mucus in your nose – constantly working to keep disease-causing microbes at bay.

For germs that get past it, there is then short-term or humoral immunity: the rapid response tailored to a specific invading threat, such as a virus, that dominates early after it arrives to try and prevent an infection from spreading to fix. This wave of defenses is spearheaded by neutralizing antibodies specifically engineered to fight whatever has entered the body.

But when that antibody response doesn’t stop Covid from taking hold and the virus enters cells so it can multiply, a third strand of protection comes into play: long-term, cellular immunity. An important part of this are memory T cells, which are also tailored to the specific threat.

“Once a virus infects cells, T cells can limit the amount of viral replication,” says Céline Gounder, infectious disease specialist and editor at KFF Health News. When a virus like Covid multiplies, it parks parts of itself in the cell’s outer membrane, which tells the host that the cell is infected. T cells—primed by vaccination or previous infection to notice these strange bits—then kick in, killing infected cells and directing the production of more antibodies. “This prevents the progression of the disease,” says Gounder.

While cellular immunity doesn’t stop an initial infection, it’s what keeps people out of the hospital, out of the intensive care unit, and out of the morgue, says Offit. “The second good thing is that T cells often live for years, decades, or life,” he says — meaning the protection they offer against serious disease can be long-lasting.

And there is a third major advantage. In Covid, some of the viral parts that land on cell membranes and attract T cells are “highly conserved” internal parts of the coronavirus – parts that are much less likely to mutate and become invisible to the immune system. The proteins that line the outside of the virus, which are normally targeted by antibodies, are much more likely to mutate, making those antibodies less effective.