Scientists don't fully understand why some vaccines (smallpox) induce strong protection that lasts a lifetime, while others require regular boosters.
The COVID-19 vaccines have proved to be the biggest weapon in our fight against the deadly novel coronavirus. However, it is unclear as to exactly how long the vaccines prevent COVID-19, if booster shots may be needed down the road, or if vaccines will need to be tweaked to fight against emerging variants of the virus.
In a report on April 2, the Centers for Disease Control and Prevention (CDC) studied almost 4,000 vaccinated healthcare personnel, first responders, and other essential and frontline workers.
They found that the messenger RNA (mRNA) vaccines developed by pharmaceutical companies Pfizer-BioNTech and Moderna prevented 80% of cases after the first dose and 90% after the second dose. The frontline workers in the study were tested for COVID-19 every week for 13 weeks.
Researchers said the dearth of positive COVID-19 tests in the study group indicates that the vaccines reduce the risk of transmission of SARS-CoV-2 by vaccinated individuals to others.
In April, both Pfizer and Moderna reported that their vaccines provided at least six months of protection.
Now, a study from researchers at Washington University School of Medicine in St Louis, published June 28 in the journal Nature, has found evidence that the immune response to such vaccines is both strong and potentially long-lasting.
What study suggests
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.
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.
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.
What baffles scientists
Scientists don't fully understand why some vaccines, such as the one for smallpox, induce strong protection that lasts a lifetime, while others, such as the vaccine for whooping cough, require regular boosters.
But many suspect that the difference lies in the quality of the germinal centers induced by different vaccines.
Germinal centers, which form as the result of natural infection or vaccination, are boot camps for immune cells, a place where inexperienced cells are trained to better recognize the enemy and weapons are sharpened.
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