The cases are already starting to emerge.
When 85-year-old Colin Horseman was admitted to Doncaster Royal Infirmary at the end of December, it was for a suspected kidney infection. But not long after, he caught Covid-19 – at the time, about one in four people in the hospital with the virus had contracted it there. He developed severe symptoms and was eventually put on a ventilator. He died a few days later.
At first glance, Horseman’s situation may seem rather typical, but no less tragic. After all, at the time of writing, at least 84,767 people have died of the disease in the UK alone. But, as his son recently explained in a local newspaper, less than three weeks earlier, he was one of the first people in the world to receive the first dose of a Covid-19 vaccine – the Pfizer-BioNTech version. He would receive the second dose two days before his death.
In fact, most vaccines require booster doses to work.
Take the measles, mumps, and rubella MMR vaccine, which is given to babies all over the world to prevent these deadly childhood infections. About 40% of people who received just one dose are not protected against all three viruses, compared with 4% of those who have had their second. People in the first group are four times more likely to contract measles than those in the second – and there have been outbreaks in places where a large proportion of people have not gone through the full MMR vaccination schedule.
“The reason people are so enthusiastic about boosters and consider them so important is that they take you to a completely different way of tuning your immune response,” said Danny Altmann, professor of immunology at Imperial College London.
How booster vaccines work
When the immune system first encounters a vaccine, it activates two main types of white blood cells. The first are the plasma B cells, which mainly focus on making antibodies. Unfortunately, this cell type is short-lived, so while your body can swim in antibodies within a few weeks, without the second shot, this is often followed by a rapid decline.
Then there are the T cells, each of which is specifically tuned to identify and kill a particular pathogen. Some of these, memory T cells, can linger in the body for decades until they hit their target – meaning immunity to vaccines or infections can sometimes last a lifetime. Most importantly, you usually won’t have much of this cell type until the second meeting.
The booster dose is a way of re-exposing the body to the antigens – the molecules on pathogens that activate the immune system – to trigger part two of the response. “You kicked all this nice stuff in the closet,” says Altmann. “So once you get your boost, you’ll have a higher frequency of memory T cells, and ditto to some extent for the size of the pool of memory B cells you have. They will also make higher quality antibodies. . “
On a second exposure to the same vaccine or pathogen, the B cells that are left beforehand can rapidly divide and create a threatening crowd of progeny, leading to a second spike in the amount of circulating antibodies.