Researchers at Washington University School of Medicine in St. Louis have found evidence that the immune response to Covid-19 vaccines, namely Pfizer and Moderna, is both strong and potentially long-lasting.

The researchers extracted cells from 14 people who received the Pfizer vaccine. Samples were obtained three weeks after the first dose, and at weeks four, five and seven. Ten of the participants gave additional samples 15 weeks after the first dose. None of the participants previously had been infected with the virus that causes Covid-19, the Xinhua news agency reported.

Three weeks after the first dose, all 14 participants had formed germinal centers with B cells producing antibodies that target a key SARS-CoV-2 protein, the virus that causes Covid-19. The response expanded greatly after the booster shot and then stayed high. Even 15 weeks after the first dose, eight of 10 people still had detectable germinal centres containing B cells targeting the virus.

"This is evidence of a really robust immune response," said co-senior author Rachel Presti, associate professor of medicine. "Your immune system uses germinal centres to perfect the antibodies so they can bind well and last as long as possible. The antibodies in the blood are the end result of the process, but the germinal centre is where it is happening."

The researchers also obtained blood samples from 41 people who received the Pfizer vaccine, including eight who previously had been infected with the virus that causes Covid-19. Samples were obtained prior to the administration of each dose of the vaccine, as well as at weeks four, five, seven and 15 after the first dose. In people without prior exposure to the virus, antibody levels rose slowly after the first dose and peaked one week after the second. People who previously had been infected already had antibodies in their blood before the first dose. Their levels shot up quickly after the first dose and peaked higher than the uninfected participants' levels.

"When we looked at the data we could see an effect," said co-first author Jane O'Halloran, an assistant professor of medicine. "If you've already been infected and then you get vaccinated, you get a boost to your antibody levels. The vaccine clearly adds benefit, even in the context of prior infection, which is why we recommend that people who have had Covid-19 get the vaccine."

Moreover, vaccination led to high levels of neutralizing antibodies effective against three variants of the virus, including the Beta variant from South Africa that has shown some resistance to vaccines.

Unlike most vaccines, the Pfizer and Moderna vaccines were created with mRNA technology, and mRNA-based vaccines provide instructions for the body to build and release foreign proteins, such as the spike protein in the case of the SARS-CoV-2 virus.

The study is published Monday in the journal Nature.