You may get more COVID-19 booster shots, but you will not receive an updated version of the vaccine.

Experts wonder if this booster campaign missed a chance to target the delta and its potential descendants.

“Don’t you want to match new strains that will circulate as closely” Dr. Cody Meissner, a Tufts Medical Centre adviser, asked Pfizer scientists to do so recently.

“I don’t understand why this isn’t delta because that’s what we’re facing right now,” Dr. Patrick Moore, a University of Pittsburgh adviser, said last week while government experts debated whether Moderna boosters were necessary. He wondered if such an option would be especially useful in blocking mild infections.

Simple answer: Pfizer approved extra doses of its original recipe last month after studies proved it works against delta. These doses could then be immediately rolled out. The FDA is now looking at evidence to support boosters of the Johnson & Johnson and Moderna vaccines.

“It’s easier to switch formulas when they’re really needed,” said FDA vaccine chief Dr. Peter Marks.

Moderna and Pfizer are betting on the future. They are already testing new doses that are customized to delta and another variation, as well as learning how to quickly modify the formula for future changes — whether it’s for today’s mutants or an entirely new one. Regulators have to decide when and how to order such a switch. This is the toughest question.

What we know so far


Even after the introduction of the delta variant, the U.S. vaccines are still effective in preventing hospitalizations and deaths from COVID-19. However, authorities hope to increase protection for those at high risk and provide more severe protection. Studies have shown that an additional dose of the original formulas boosts virus-fighting antibodies that fight infection. This includes antibodies that target delta.


Vaccines attack the spike protein that coats coronavirus. According to Richard Webby, a virus expert at St. Jude Children’s Research Hospital, mutations in this protein made delta more contagious. However, the immune system doesn’t see any difference.

John Wherry, University of Pennsylvania immunologist, stated that there is no guarantee that a delta-specific booster will protect more people. It would have been inconvenient to delay the release of boosters to those who are deemed to be in need by waiting for research to answer that question and, if necessary brewing up updated doses.

However, delta, the predominant version of the virus, is the most prevalent in the world. It almost certainly will become a common ancestor for any future evolutions in an unvaccinated population, according to Trevor Bedford, a genetics expert and biologist at the Fred Hutchinson Cancer Research Center.

He said that a delta-updated vaccine “would help to provide a buffer for those additional mutations.” Howard Hughes Medical Institute pays Bedford, and also supports The Associated Press Health and Science Department.


Moderna and Pfizer vaccines contain messenger RNA, a small piece of genetic code that instructs the body how to make harmless copies the spike protein to train it to recognize the virus. The only way to update the formula is to replace the original genetic code by mRNA in order for the spike protein to be mutated.

The companies tested tweaked doses against the South African beta variant of the virus, which has proven to be more resistant than the delta variant. The updated shots were found to produce potent antibodies in lab tests. However, the beta variant was not widely distributed.

The companies are currently conducting studies on fully vaccinated individuals who have agreed to test a booster dose that was adjusted to match the delta. Moderna’s studies include shots that provide protection against multiple coronavirus strains, much in the same way as today’s flu vaccines.

Although mRNA vaccines are the easiest to modify, other vaccine manufacturers are also looking at ways to alter their formulas if needed.


Moderna’s Dr. Jacqueline Miller explained to an FDA advisory panel that the company is currently studying variant-specific boosters to determine if they have any advantages and be prepared if needed.

Penn’s Wherry stated that it is important to analyze how the body reacts when given new shots. The immune system has a tendency to “imprint,” a stronger memory for the first strain of virus it encounters. This raises questions about whether the body would react differently to a booster that causes a temporary increase in antibodies. Or if the larger goal is to create a stronger and longer-lasting response that may be better prepared for the next virus strain.


Webby asked, “What’s the tripping point?” He is part of a World Health Organization network which tracks the evolution of influenza. Unfortunately, a lot of the information that will be required to make those decisions can only be learned through experience.

Bedford stated that now is the right time to determine what drop in vaccine effectiveness would cause a formula change. This is exactly as happens with flu vaccines every other year.

This is important, not only in the event of a significantly worse variant. Bedford, like many scientists, expects that the coronavirus will eventually become a worldwide crisis and a threat every winter. This could mean regular boosters or even the flu shot yearly.

Wherry also noted that timing between shots is important.

He said that boostability could actually be improved by having longer intervals between stimulation. Although scientists have a lot of information about coronavirus, the story is still far from over and we don’t know the ending.