Regulators (the MHRA in the UK) would need to see safety and efficacy data in children before any vaccine’s use could be authorised in this age group.

Before considering whether it makes sense to vaccinate children, we need to think about why we might wish to do this.

Covid-19 is a serious disease. The consequences in terms of mortality and serious illness (hospital admissions, ICU admissions…), and long-term sequelae (e.g. “Long Covid”) are considerable – much greater than, for example, for influenza.

We need to reduce this “burden of disease” to a tolerable level.

Vaccination is never 100% effective. Some people, even if fully vaccinated, will become ill, and some will die.

Lockdown” works. The UK was one of the first countries to experience the more transmissible B.117 variant; and it caused a dramatic increase in cases in December and January (exacerbated by Christmas and New Year mixing). Lockdown has driven the case numbers down.

But we are all tired of locking down. We want to see our friends and our relatives. And the big hope of getting there is vaccination.

But will vaccines deliver this? In order to maintain the burden of Covid-19 disease at a tolerable level we need to get the effective R number (Rt) below 1, and keep it there.

Anything else will mean an exponential increase in case numbers, disease in those who are unvaccinated or inadequately protected through vaccine or previous illness, massive amounts of viral replication, and an increased risk of mutations potentially leading to more virulent forms or transmissible forms of the disease, and very likely leading to the generation of viral escape variants, against which current vaccines are ineffective.

Will vaccines do enable us to get Rt below 1 without other restrictions?

The answer is – they might. With the original variant, R0 was about 3. With the more transmissible variants, it could be up to 5, maybe even more. For vaccine-induced herd immunity to work, you need more than all-but-one in (the R0 number) to be immune, unable to be infected and transmit the disease – more than 2 in 3 for the original variant, more than 4 in five for the more transmissible variants… And the vaccine is unlikely to be 100% effective at preventing infection and transmission, so the proportion of the population we’d have to vaccinate is likely to be well over 90%. We don’t even know, yet, whether we will be able to vaccinate enough people to get the proportion of the population that has sufficient to be unable to be infected and transmit the disease high enough to keep Rt below 1 through vaccination alone: you can’t vaccinate more than 100% of the population. If the proportion you need to be unable to be infected and transmit the disease is greater than the proportion in whom the vaccine can induce this level of immunity, vaccination alone will never be able to keep Rt below 1.

What is clear, is that the proportion of the population that we will need to vaccinate to do this (if it can be done at all) will be high. And it needs to include everybody who would be able to be infected and transmit the disease.

There is now ample evidence that children can be infected and infectious. They are much less likely to be symptomatic, or to have serious disease; and they (particularly younger children) may be less likely to be infectious. (This fits with transmission being via aerosols – the same applies to TB.) But if we are to reach herd immunity through vaccination, we will definitely need to vaccinate everybody who might be infectious, and that will include children.

In terms of priorities, older children will be a higher priority, because they are more likely to infect others. This is both because they are larger, more adult-like, and therefore more likely to transmit the disease to those that they meet; and because they generally mix with more people than younger children, and therefore have more opportunities to infect others.

But, given the difficulty of getting enough people immune through vaccination to keep Rt below one without restrictions on contact, we will almost certainly need to vaccinate all children.

One ray of hope is that reinfection or infections after having been vaccinated are likely to be much less severe than primary infection: the partial immunity acquired will “attenuate” the severity of the illness. But we will still need to get and keep Rt below 1.

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Multiple research groups worldwide are still collecting data on vaccines in teenagers and children and the results may come through at different times throughout the year. The MHRA will not yet have enough data on teenagers and children to give approval for any one vaccine, and it’s likely more than one trial will be needed before approval for use in under 18’s is given.

It is also unlikely that JCVI would implement a programme for teenagers or younger children without this approval unless there was immediate concern about increases in transmission in the UK of a new variant for which immunising young people was considered essential. This is itself less likely over the summer months.

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