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The Covid-19 vaccine most well-liked by Pfizer and BioNTech has fewer negative effects than the primary

In this photo illustration, the American multinational pharmaceutical company's Pfizer logo is displayed on a smartphone with a computer model of the COVID-19 coronavirus in the background.

Budrul Chukrut | SOPA pictures | Getty Images

Pfizer and BioNTech surprised many industry watchers on July 27 when they announced they would conduct a large-scale study of a vaccine against Covid-19. The surprise? The vaccine that was to be tested in a study of 30,000 patients wasn't the one the companies presented data on July 1.

The reason, the companies said, was that a second vaccine appeared to produce a similar immune response, but fewer side effects. On Thursday, they released the results of all 332 people who had received either a vaccine called Vaccine B1 or B2 – and in fact, B2 recipients had significantly fewer adverse events related to the vaccine.

"The better the vaccine is tolerated, the more I believe it will promote widespread vaccination acceptance," said William Gruber, senior vice president of clinical research and development for vaccines at Pfizer. "Both would have been great candidates. We were fortunate that B2 was actually satisfied with both a favorable immune profile and fewer responses."

The study tested doses of each vaccine ranging from 10 micrograms to 100 micrograms. The 30 microgram dose of B2 is being further developed in clinical studies.

With the original vaccine, called BNT162b1 or B1 for short, patients between the ages of 18 and 55 had adverse events that were believed to be related to the vaccine dose of 30 micrograms 50% of the time. People between 65 and 85 years of age had adverse events 16.7% of the time.

For the second vaccine, BNT162b2 or B2, patients between 18 and 55 years of age experienced 16.7% of adverse events believed to be vaccine related and no adverse reactions were reported that were believed to be related became that they were related to the vaccine 65 and 85.

Both vaccines use mRNA – the genetic messenger with which the body converts the DNA code into proteins – packaged in a fat capsule, a so-called lipid nanoparticle, which enables it to enter cells. The mRNA instructs the cells to make a protein, which then activates the immune system. For the B1 vaccine, the mRNA encodes that part of a protein on the SARS-CoV-2 virus that binds to a receptor on human cells in order to gain access to them. The B2 vaccine makes all of the protein known as the spike protein.

Using the full spike protein can allow the immune system to find more ways to recognize and attack the virus. Chemical modifications to the mRNA can also explain part of the difference. Although patients were given the same weight dose with each vaccine, the B2 vaccine would contain fewer particles because the full-length mRNA is heavier.

The side effects tracked were mainly those that would be expected from an injection of the vaccine, including pain at the injection site, fever, chills, headache, and muscle or joint pain. No older adult who received B2 reported redness or swelling at the injection site.

The average level of antibodies to the virus in older adults was only 41%, compared to younger participants. However, it was still higher than the amount of antibodies seen in recovered patients, the authors said.

All of the patients in the B2 study were white and not Hispanic, with more older women participating than older men. The younger patients had a median of 37 years old, while the older patients had a median of 69 years.

Pfizer has said that some data from its large study of the B2 vaccine could be in as early as October.

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