Pretend it didn't happen – expert advice on how to behave after receiving a single dose of any of the Covid-19 vaccines. //
When the immune system first encounters a vaccine, it activates two important types of white blood cell. First up are the plasma B cells, which primarily focus on making antibodies. Unfortunately, this cell type is short-lived, so although your body might be swimming in antibodies within just 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 tailored to identify a particular pathogen and kill it. Some of these, memory T cells, are able to linger in the body for decades until they stumble upon their target – meaning immunity from vaccines or infections can sometimes last a lifetime. But crucially, you usually won't have many 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 trigger the immune system – to initiate part two of the response. "You've kicked in all this fancy stuff," says Altmann. "So, once you've had 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'll have. They'll also be making higher quality antibodies."
On second exposure to the same vaccine or pathogen, the B cells that remain from before are able to rapidly divide and create a menacing throng of descendants, leading to a second spike in the amount of antibodies circulating.
The second dose also initiates the process of "B cell maturation", which involves selecting the immature ones with the best receptors for binding to a particular pathogen. This happens while they're still in the bone marrow – where white blood cells are made – and afterwards they travel to the spleen to finish developing. This means B cells are not only more numerous afterwards, but the antibodies they produce are better targeted.
