Some vaccines last a lifetime – so why do we need COVID-19 boosters?

Why is it that some vaccines you only need once, but for others you need to get boosters?

Most countries aim to give two jabs against measles during childhood, for example, and then that’s it for life. But multiple jabs are given against tetanus – the US recommends one every 10 years for adults. And some countries are now giving out a third or even fourth dose of the COVID-19 vaccine in the space of little over a year after studies suggested effectiveness was waning.

The answer lies in a number of factors.

Vaccines aim to mimic natural immunity. By exposing the body to harmless imitations, they create an immune memory, teaching the body to recognize infections from disease-causing pathogens. When an infection is recognized, an immune response is mounted, with antibodies latching onto the invaders and preventing them from causing illness. T-cells, which can attack pathogen-infected cells, are also crucial to the body’s defences.

All vaccines, including the COVID-19 vaccines, produce an immune response, but the strength of that response, and the duration of the resulting antibodies, varies depending on the effectiveness of the vaccine.

And different levels of antibodies are required to protect against different viruses and bacteria.

Measles antibodies tend to be fairly long-lasting, so there are sufficient levels circulating in the body to ward off infection for years after vaccination. Research is still ongoing into the level of antibodies required to protect against COVID-19.

Added to this is the extent to which the virus or bacteria mutates or evolves. A rapidly evolving infectious agent may become able to evade the body’s defences if it looks sufficiently different to previous incarnations or finds new methods of attack. And creating a vaccine against a rapidly evolving enemy is like trying to hit a moving target.

We see examples of this each year with flu season. As the influenza virus replicates, small changes can emerge in its genetic make-up. This can lead to changes in the virus’s surface proteins, which are key to our immune system’s ability to recognize infection and trigger a response. This so-called antigenic drift usually produces viruses fairly similar to their predecessors, and antibodies created against one flu virus will probably still recognize and respond to similar viruses.

However, these small changes can accumulate, hampering our immune response to newer virus iterations and making a person susceptible to flu infection once again. For this reason, flu vaccine “recipes” are tweaked each year to make them as effective as possible against the viruses circulating at the time.

The various coronavirus vaccines on the market were released as soon as they were shown to be safe and effective. And scientists are continuing to monitor long-term immunity, including antibody levels, in people that have received jabs.

The approved vaccines use different methods to initiate an immune response. Some are based on a non-replicating adenovirus, a common virus that can cause colds and other illnesses. Others use mRNA, a molecule that carries genetic code from DNA and contains no virus at all.

Vaccines based on mRNA are a comparatively new technology, and research is still being carried out into their longevity.

The length of immunity may also vary from person to person, depending on factors such as age, pre-existing conditions and any medications they might be taking.

Ongoing studies suggest that our current COVID-19 vaccines may be less effective against symptomatic disease caused by the Omicron variant compared with previous virus forms. But people who have had a booster dose are better protected than those who have had only their initial course of two jabs. And while protection does wane, the defence offered against severe disease is still strong.