By Kristen V Brown

Why do vaccines wear off so quickly?

In this week’s edition of the Covid Q&A, we look at the waning effectiveness of vaccines. In hopes of making this very confusing time just a little less so, each week Bloomberg Prognosis picks one reader question and puts it to experts in the field. This week’s question comes to us from Ruth in Chicago. She asks:

I keep wondering why the Covid vaccine’s effectiveness wanes so quickly. Is it the messenger RNA technology? Something else?

The reality of life after vaccination has looked a little different than we imagined. There was that glorious period after vaccines first rolled out when it seemed like the pandemic was at long last over and we could just slip right back into our old pre-Covid lives. SARS-CoV-2, though, is a wily virus. It keeps evolving around our defenses, with new variants like omicron rendering vaccines and treatments less effective.

“The situation with the Covid-19 vaccines actually is similar to the situation that we have annually with the flu vaccine,” says Katrine Wallace, an epidemiologist at University of Illinois at Chicago.

It’s not that the effectiveness of the shots is wearing off so much as the virus is evolving to be better at evading the first-line defenses of vaccines. 

“The available vaccines were created for the original 2019 ‘wild-type’ SARS-CoV-2 virus,” says Wallace. “The virus has undergone multiple mutations since then, and several new variants have circulated globally (alpha, beta, delta, omicron). Viruses mutate all the time, and a viral variant occurs when a mutation causes the virus to behave differently, either to become more transmissible, cause more severe disease or cause decreased effectiveness of available vaccines or medications.”

A Clinic Administers Fourth Doses Of Covid-19 Vaccine Booster
When we might need more boosters is still very much up in the air. Photographer: Hannah Beier/Bloomberg

In the case of SARS-CoV-2, those mutations have occurred largely on the spike protein, the crown-shaped part of the virus that helps it invade the body’s cells. The spike is also the target of vaccines and other Covid therapies like monoclonal antibody drugs. So as the virus mutates, it gets better at evasion.

However, so far the virus has appeared adept only at getting around the first-line defenses of vaccines — that’s why hospital rates and deaths haven’t increased at the same rate of infections, even in the face of the hyper-contagious omicron variant. Other immune defense systems, such as T-cells spurred by vaccines, still appear effective at fighting off Covid. 
 
“But even for those endpoints, it is not 100% effective,” says Wallace. “In the case of omicron, it is so transmissible that even with an effective vaccine against severe disease, during large surges we will still see severe cases happening because of the high number of infections.”

For that reason, vaccine manufacturers are investigating omicron-specific boosters.

At present, immunocompromised people and those over age 50 are eligible in the U.S. for a second booster shot. Just this week, the Food and Drug Administration held a meeting of its panel of outside vaccine experts to discuss future boosters. The main takeaway: There isn’t an obvious answer to when more boosters will be necessary. 

People often compare Covid with influenza, which also mutates frequently, requiring new flu shots every year. Those flu shots aren’t made with mRNA technology but still are susceptible to viral changes. Also, Wallace points out we have all had a lifetime of exposure to the flu.

“A virus can spread and mutate a lot faster within a population of completely susceptible hosts,” she says. 

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