A LEADING US virologist says he believes we could safely wait "three to five years or longer" before administering Covid booster vaccinations.

Dr Paul Offit, a co-inventor of the life-saving rotavirus vaccine, also said he was "all for" giving patients incentives such as free beer to boost vaccine uptake as he warned that countries such as the UK and US face winter surges unless coverage is maximised.

Dr Offit, a professor of vaccinology at the University of Pennsylvania and a member of the FDA's vaccines committee, said he was worried that in the US there is still "a solid 30-40% of this population that could be vaccinated but are choosing not to be vaccinated".

The academic was interviewed by Edinburgh Napier journalism lecturer, Alex Kocic, for a podcast broadcast in his native Serbia.

The English language recording was shared exclusively with the Herald before being dubbed.

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It comes as the UK is experiencing its fastest growth in coronavirus cases since February as a result of the spread of the Delta variant first detected in India.

This has led to a push to give as many people as possible second vaccine doses and calls to delay the roadmap out of lockdown until the impact of vaccinations on hospital admissions and deaths is clearer.

HeraldScotland: Covid cases in the US are averaging 44 per million per day, compared to 80 per million per day in the UKCovid cases in the US are averaging 44 per million per day, compared to 80 per million per day in the UK

Research indicates that a single dose of either Covid vaccine is only 33 per cent effective at preventing symptomatic infection caused by the Delta variant, with preliminary data published last week showing that the levels of neutralising antibodies - proteins which block the virus from entering cells - are five times lower against the Delta variant than the original Wuhan strain even after two doses of the Pfizer vaccine.

Scientists at the Francis Crick Institute, who shared their findings in the Lancet, said the antibody response was lowest among the very elderly and declined over time, providing evidence of the need to deliver booster jags in Autumn.

HeraldScotland:

However, Dr Offit stressed that immunity could still be present even when antibodies have faded in the blood.

He said: "If you look at the immune response - specifically cellular immunity induced by either natural infection or vaccination - you find fairly high frequency of these so-called memory T cells like T helper cells which make healthy cells make antibodies, or cytotoxic T cells which kill virus-infected cells. That's a good sign. That usually predicts longer-term immunity.

"If the goal is to prevent severe, critical disease, and that we will need a booster only if the immunity drops below that level necessary to protect against severe critical disease - if that's the goal - then I think it would be likely that we wouldn't need a booster for a few years. Maybe longer - may be every three to five years, or longer."

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The emergence of the more transmissible Delta variant has added to the pressure to ramp up vaccinations as the more contagious a virus is, the higher the threshold for herd immunity.

Current estimates suggest that roughly 75 per cent coverage may have been sufficient when the Alpha ('Kent') variant was dominant, but may require 83% uptake or more across the total population to combat the Delta variant.

"The so-called reproducibility index or contagiousness index of that [Delta] virus is high and if it's high then you need a greater percentage of the population to be immune if you're going to significantly stop the spread," said Dr Offit, adding that people had become "a little complacent" about the dangers due to summer.

He said: "At its heart, this virus - SARS-CoV-2 - is a winter respiratory virus.

"If you look at deaths for example last year, when the virus first came into the United States and started killing people in March the death rate went up to a thousand deaths a day, 2000 deaths a day, 2500 deaths a day, then as we entered summer it came down and stayed down pretty much up until we hit November/December.

"Well, last summer we had a population that was fully susceptible - we didn't have a vaccine yet still the number of deaths came down because it is at its heart a winter virus.

"So the numbers right now in terms of cases and deaths are down, but I think that unless we get an adequate percentage of the population protected then come next winter you're going to see a surge again."

HeraldScotland: The UK is slightly ahead of the US on first dose vaccine coverageThe UK is slightly ahead of the US on first dose vaccine coverage

The US is currently reporting 44 cases per million against 80 per million - and rising - in the UK.

In the US, where virus rates are the lowest in over a year, 51% of the total population have had at least one vaccine dose compared to 60% in the UK.

Dr Offit said it was wrong in his view to treat Covid vaccination as a "personal decision".

"If you choose not to get a tetanus vaccination and then you get tetanus, that's a personal decision," said Dr Offit. "Nobody's going to catch tetanus from you - it's not a contagious disease.

"This is a contagious disease. This is a decision that you're making for everyone with whom you come into contact.

"In the United States there are hundreds of thousands of people who can't be vaccinated because they're getting chemotherapy for cancer, because they're getting biologic for chronic diseases.

"They depend on those around them to protect them, and I think it's a societal responsibility that if you're going to walk around outside you have to realise that if you're unvaccinated, you're putting yourself and others at risk."

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He cited the Los Angeles Dodgers baseball team, who have ring-fenced a section of their stadium as "vaccinated-only", as an example of societal interventions which should be put in place to encourage uptake.

In New Jersey, vaccine recipients have been rewarded with free beers, while Washington state - where cannabis is legal - launched its "joints for jabs" incentive scheme this week and in Ohio residents who take up the vaccine are automatically entered into a lottery to win $1 million (£700,000).

HeraldScotland:

"You would like to think that people would get the vaccine [because] it would give them a chance to avoid the suffering, hospitalisation, and death that's caused by this virus, that that should be enough of an incentive - but apparently it isn't," said Dr Offit.

"So if a free beer works, I'm all for it. But for some it may be that nothing is going to convince them to do it and then you have to ask yourself the question that, if a significant percentage of the population is choosing not to get vaccinated and that allows the virus to continue to spread, then what do you do?

"I think that's when you move to mandates."

with additional reporting by Alex Kocic