The issues are already starting to emerge.
When Colin Horseman, 85, was admitted to Doncaster Royal Infirmary in late December, it was for a suspected kidney infection. But shortly afterwards, he caught Covid-19 – at the time, about one in four people in the hospital contracted the virus. He developed severe symptoms and was eventually put in a ventilator. A few days later he died.
At first glance, Horseman’s situation seems quite typical, though it is no less tragic. After all, at least 84,767 people in the UK alone succumbed to the disease at the time of writing. But, as his son recently explained in a local newspaper, he was less than three weeks earlier among the first people in the world to receive the initial dose of a Covid-19 vaccine – the Pfizer-BioNTech version. He had to receive the second dose two days before his death.
In fact, most vaccines require booster doses to work.
Take the MMR vaccine – measles, mumps and rubella – given to babies around the world to prevent these deadly infections in children. About 40% of people who received only one dose were not protected against all three viruses, compared to 4% of those who had their second dose. People in the former group are four times more likely to catch measles than those in the latter – and there have been outbreaks in places where a large proportion of people have not completed the full MMR vaccination schedule.
“The reason people love boosters so much and consider them so vital is that they send you to this other kind of measurement mode of your immune response,” says Danny Altmann, professor of immunology at Imperial College London.
How booster vaccines work
When the immune system first gets a vaccine, it activates two important types of white blood cells. The first place is the plasma B cells, which mainly focus on making antibodies. Unfortunately, this cell type is short-lived, so although your body can swim in antibodies within a few weeks, it is often followed by a rapid decline without the second shot.
Then there are the T cells, each of which is specifically adapted to identify and kill a specific pathogen. Some of these, memory T cells, can linger in the body for decades until they reach their target, which means that immunity to vaccines or infections can sometimes last a lifetime. But very important is that you usually do not have much of this cell type until the second encounter.
The booster dose is a way to re-expose the body to the antigens – the molecules on pathogens that trigger the immune system – to start part two of the reaction. “You kicked in all this flashy stuff,” Altmann says. Once you get a boost, you have a higher frequency of memory T cells and this to some extent for the size of the pool of memory B cells you will have. They will also make higher quality antibodies. “
Upon second exposure to the same vaccine or pathogen, the B cells previously left can divide rapidly and create a threatening crowd of offspring, leading to a second increase in the amount of antibodies circulating.