Vaccine race just not the same for children

In the race to develop vaccines against Covid-19, vaccines will be assessed in clinical trials that comprise all key age groups including children. However, kids don’t join the trials until after the adults have gone first.

Young children and infants are some of the most vulnerable members of our society, they are dependent on adults to make the best decisions for them. They are often both healthy, and at the same time more susceptible to a range of diseases. The bar for vaccine safety is set very high and this is because most people receiving vaccines are healthy. All vaccines are first tested on adult volunteers.

Clinical studies for Covid-19 vaccines are taking place around the globe and are generally recruiting healthy adults aged 18 and over. This is the normal and appropriate process when testing a new vaccine. First, you test it in a few healthy young adult volunteers aged around 18 to 45 to check out the best dose and investigate the safety. This is the case even if your most at-risk people are older, younger, or have underlying medical conditions. Testing in these groups comes later. Once a vaccine has been tested in the healthy adult volunteers, then other age groups - younger and older - can be included in a step-wise process.

In the case of Covid-19 vaccines, the age is already being extended upward which is logical and ethical because the highest risk group for serious Covid are older. The good news is that the RNA vaccines under development are looking like they generate a nice immune response in quite old people. The viral-vectors may be less efficient in the elderly.

But what about the kids? Children get and spread Covid-19 so they will need to be vaccinated both to protect them and also to prevent spread. But in terms of testing, trials in these younger age groups would not take place until a good vaccine safety profile has been established. And vaccines will not be given to children outside a clinical trial until there is relevant data specific to their age group available.

So do we expect different vaccine responses in children? Interestingly, in general, it is these younger age groups that have the best responses to vaccines, although that is dependent on the type of vaccine being used. We give a higher dose of tetanus-diphtheria, whooping cough vaccine to prime infants and a lower dose to boost adults. Younger people (under 15 years) who receive the HPV vaccine achieve the same result as an adult getting three doses. Even new-born infants can mount a pretty good response to many vaccines and this improves over the first years of life. The infant immune system is born rearing to go and develops as it is exposed to bugs.

As we age, our immune system becomes less able to respond to both new infections and many vaccines. For example, our current influenza vaccines have many limitations in this older group. Conversely, and fortunately, there is a new vaccine against shingles that uses a similar approach to some Covid-19 vaccines and it is working extremely well in even frail older people. It is not a forgone conclusion that a given vaccine won’t work well in certain groups.

Once trials do begin on younger people there are unlikely to be safety issues identified specific to that age group, there is no previous evidence to suggest that this would be the case. However, this would still be assessed rigorously regardless. It is at this phase II stage where a lower dose may be found equally as effective and also less reactogenic (i.e. producing unwanted side effects). This can be assessed through measuring the immune response and if the response is strong but there are too many sore arms then a lower dose may be selected for that group.

We may well find that the first vaccines we get access to will be initially licensed for use in adults with that being expanded to children and infants as data comes to hand. The vaccines will not be used in children without clinical trial data to support it.

With regard to creating herd immunity through vaccines, we do not yet know which vaccines will prevent transmission as well as disease, and which will be effective in older people. If we had a scenario where we had a vaccine that prevented transmission but did not work well in older people then we may target those around the most vulnerable and those most likely to spread the infection. In the case of flu, vaccinating children can have a best effect on older people because they do the most spreading. Conversely, if we get a vaccine that works well in all age groups we can start giving it to everyone in order of their risk.

Ultimately, we will need to wait and see what we are working with when the time comes.