The cases are already beginning to emerge.
When Colin Horseman, 85, was admitted to the Doncaster Royal Infirmary in late December, it was on suspicion of kidney infection. But not long after he took Covid-19 – at the time, about one in four people in the hospital with the virus had contracted it. He developed severe symptoms and was eventually put on a respirator. A few days later, he died.
At first glance, Horseman’s situation may seem quite typical, though no less tragic for that reason. After all, at least 84,767 people have already succumbed to the disease in the UK alone at the time of writing. But, as his son recently explained in a local newspaper, less than three weeks earlier, he had been one of the first people in the world to receive the initial dose of a Covid-19 vaccine – the Pfizer-BioNTech version. He was due to receive the second dose two days before his death.
In fact, most vaccines require booster doses to work.
Get the MMR vaccine – measles, mumps and rubella – which is given to babies around the world to prevent these fatal infant infections. About 40% of people who received only one dose are not protected from the three viruses, compared with 4% of people who received the second. People in the first group are four times more likely to get measles than those in the second – and there have been outbreaks in places where a high proportion of people have not completed the full MMR vaccination schedule.
“The reason people like reinforcements so much and consider them so vital is that they kind of send you into another type of fine-tuning of your immune response,” said Danny Altmann, professor of immunology at Imperial College London.
How Booster Vaccines Work
When the immune system encounters a vaccine for the first time, it activates two important types of white blood cells. First are plasma B cells, which are mainly concentrated in the production of antibodies. Unfortunately, this type of cell is short-lived, so although your body may be swimming in antibodies in just a few weeks, without the second injection, this is usually followed by a rapid decline.
Then there are the T cells, each of which is specifically adapted to identify a specific pathogen and kill it. Some of them, memory T cells, are able to stay in the body for decades until they find their target – meaning that immunity against vaccines or infections can sometimes last a lifetime. But, crucially, you generally will not have many such cells until the second meeting.
The booster dose is a way to reexpose the body to antigens – the molecules of pathogens that activate the immune system – to initiate part two of the response. “You started all these fancy things,” says Altmann. “So once you get your boost, you’ll have a higher frequency of memory T cells and ditto, to some extent, the size of the memory B cell pool you’ll have. They’ll also be producing higher antibodies quality. “
In the second exposure to the same vaccine or pathogen, the remaining B cells are able to divide quickly and create a threatening crowd of offspring, leading to a second increase in the amount of circulating antibodies.