Professor Adam Finn, from the University of Bristol who sits on the JCVI, said more than one study is needed before decisions are made about extending the vaccination programme to youngsters.
While children are unlikely to fall ill with Covid-19, they do play a role in transmitting the virus.
The University of Oxford is currently carrying out a clinical trial on children to test the safety and efficacy of its vaccine in younger age groups, with initial results expected in the summer.
The trial is working with partner sites in London, Southampton and Bristol and includes around 300 youngsters aged six to 17.
Responding to reports that children could be vaccinated from August, Prof Finn told Good Morning Britain: “As far as I know there has been no decision made to immunise children starting in August, or indeed any decision been taken to immunise children at all at this point.
“But it’s certainly something that we might need to do.”
He said results from more than one study are needed, adding: “If it does turn out to be necessary to immunise children, I think it is more likely that we would prioritise teenagers over younger children, simply because the evidence we have at the moment is that transmission of the virus is more likely to occur from and between teenagers who are a little bit more like adults.
“I think what we need to learn before that (is) what proportion of the population we need to immunise in order to get effective herd immunity and to suppress circulation of the virus.
“In order to do that, we need to have a clear understanding of how efficiently the vaccines actually interrupt infection and transmission, and that evidence is still on its way at the moment.”
Referring to the aim to have the adult population vaccinated before the end of July, Prof Finn said: “During that time we will see what goes on with variants, with the circulation of the virus, and then we’ll be able to make a decision whether children need to be immunised – we clearly won’t want to do that unless it’s necessary.
“But if it is necessary we will by then know whether the vaccines are entirely safe and effective and we’re giving the right dose and so on, so that we go forward with that later in the year.”
Prof Finn said more studies are forthcoming on how vaccines work in children, adding that “in order to establish that vaccines can safely be used in children, we need to do that”.
Currently, only children at very high risk of severe infection are offered a jab.
A spokesman for the Department of Health said: ”While clinical trials are under way to test the efficacy and safety of Covid-19 vaccines in children and young adults, these trials have not concluded yet.
“We will be guided by the advice of our experts on these issues including the independent JCVI.”
It came as Home Secretary Priti Patel praised pharmaceutical firms after Boris Johnson privately told Tory MPs the UK’s vaccine programme has been successful because of “capitalism” and “greed”.
She told Sky News: “The Prime Minister always acknowledges the strong success we’ve had in terms of the vaccine, not just the rollout, which is incredible, but also our ability as a country to develop the vaccine, the role that pharmaceutical companies and science and technology has played in that. And, actually, I think that speaks to a great strength we have as a country.”
Ms Patel did not rule out border measures remaining in place over the summer to prevent new coronavirus variants.
She told BBC Radio 4’s Today programme: “We rule nothing out in terms of the approach we take when it comes to infection control and the safety and security of our public from this virus. We will take all measures basically to protect our country and our citizens from new variants.”
The Prime Minister is set to be grilled by senior MPs over his handling of the pandemic later when he appears in front of the Liaison Committee, made up of Commons select committee chairs.
It will take place after Mr Johnson’s weekly session with Labour leader Sir Keir Starmer at Prime Minister’s Questions in the House of Commons.
Elsewhere, Belgian MEP Philippe Lamberts said he was angry about AstraZeneca’s alleged “inability to deliver, combined with a form of arrogance towards the EU as a customer”.
He repeated claims that the AstraZeneca vaccine is linked to blood clots despite the European Medicines Agency saying no causal link with the vaccine had been found.
And he told the Today programme that AstraZeneca was to blame for the EU export row rather than strained relations between the bloc and the British Government.
“We are in this position because of the inability to fulfil promises of a vaccine manufacturer – in this case AstraZeneca,” he said.
Pressed on whether the UK-based vaccine-maker is entitled to accuse the EU of orchestrating a smear campaign against it, he admitted some politicians may have “instrumentalised” their citizens’ concerns to “do some AstraZeneca bashing”.
He continued: “AstraZeneca is a company that is not straightforward, that cannot be relied upon.
“They commit, they decommit, then they decommit on their new commitments without any warning.”
Asked about the fact that the EU has millions of AstraZeneca doses that are not being used, he admitted member states have a poor track record on their vaccine rollouts.
But he added: “We obviously have also to do our job, but that does not exonerate suppliers of fulfilling their commitments regardless of what the customer does with their wares.”
Jane Halton, co-chairwoman of the Covax initiative – which is working to provide vaccines for low and middle-income countries – said holding doses of vaccine “hostage” is “not desirable”.
Asked about the threat to block vaccine exports from the EU, she told Times Radio: “We’d say that will be extremely regrettable.
“The fact that you have vaccine production inside your borders and then you choose to hold hostage doses of vaccine that have been otherwise committed elsewhere is not a situation any of us want to see.”