Covid 19: Why do Covid boosters offer greater protection against Omicron?

A syringe is prepared with the Pfizer Covid-19 vaccine. Photo / AP

Studies show three jabs are more effective than two as manufacturers work on variant-specific vaccines.

In the race to protect their populations against the Omicron coronavirus variant, countries including the UK and US, are rolling out extensive booster programmes after data showed additional jabs provided more protection against the new strain than two shots.

The data — from the University of Oxford — on Monday showed levels of neutralising antibodies that attacked the virus fell against Omicron, compared with Delta, in people who had taken two doses of the vaccine it developed with AstraZeneca, or of the BioNTech/Pfizer shot.

The substantial fall in antibodies, Oxford said, "suggests that while there is no evidence of increased potential to cause severe disease, or death, increased infections in previously infected, or vaccinated individuals may be likely".

With early hospital data suggesting Omicron is more transmissible than the dominant Delta variant, boosters are looking more beneficial than ever.

How do boosters work?

Covid-19 booster vaccines familiarise the body's immune system with the spike protein — the exposed part of the virus that binds with human cells. They enable the immune system to get better at recognising and responding to the real virus if it hits.

Boosters increase levels of antibodies and T-cells, which strengthen the immune system's response to the virus, according to Charles Bangham, professor of immunology at Imperial College London.

"The purpose of the booster is to increase the strength of the antibody and T-cell immunity, so that if you are reinfected, it becomes a trivial infection: you are less unwell, and you are less likely to transmit it to others," he said.

Teresa Lambe, an Oxford professor who helped create the university's Covid-19 vaccine and co-authored Monday's study, said a third dose helped "solidify or make a stronger memory immune response".

"That means you're in a better position for your immune response to kick in when you see the virus, so you're better protected," she said.

Why are they much better at protecting against Omicron than two doses?

The current Covid-19 vaccines were designed to tackle the original strain of Sars-Cov-2, which first emerged in the Chinese city of Wuhan. But a third dose boosts the immune system so significantly that it creates a much better army of antibodies to swarm the virus — even if it has mutated considerably.

The booster provokes a "burst of action" which is much quicker and larger than for the initial doses, said Peter English, an expert who used to work for Public Health England.

"Larger amounts of antibodies in the bloodstream can still somewhat neutralise the different variants," he said.

Early data from a lab test show that a third dose of the Pfizer vaccine resulted in a 25-fold increase in the neutralising antibodies that attack the virus against Omicron, a similar level of protection provided by two doses against the Wuhan strain.

And UK Health Security Agency data from "real world" cases of Covid-19 caused by Omicron suggest the boosters given in the UK — Pfizer and Moderna — topped up protection to make them 70 to 75 per cent effective against the variant.

How long does it take for the protection to kick in?

The UKHSA data measured efficacy from two weeks after the booster and Pfizer's trial data — which was collected before Omicron emerged — showed that the levels of antibodies, which fight the virus, soared seven days after a third jab was administered.

But some suspect extra protection may come even earlier than that. English said boosters for the virus that causes whooping cough had been shown to have an impact within 48 hours. "I can't see why Covid-19 would be that different. The booster response . . . can swing into action very, very quickly and very rapidly produce antibodies," he said.
Are variant-specific boosters being developed?

Vaccine manufacturers are already tweaking their designs to prepare shots tailored to the Omicron variant. Pfizer said it could have an Omicron booster available by March, because mRNA technology can be used to rapidly produce a variant-specific vaccine.

But its availability broadly depends on how long a trial will take. Regulators are likely to require another trial that would take at least two months.

Penny Ward, visiting professor in pharmaceutical medicine at King's College London, said the decision to switch would depend on how well the current boosters were protecting people against severe disease and death, and the antibody levels of those vaccinated with a tweaked vaccine.

"If they show that giving a boost with the tweaked variant is much better than the booster that would imply superior clinical effectiveness," she said.

Airfinity, the health analytics firm, predicts that, in a best-case scenario, only 6bn doses of an Omicron-targeted shot will be available by October 2022. If vaccine makers only switched half the production, that target would be reached in January 2023 at the earliest.

Will we need to boost forever?

A scientific consensus has not yet emerged about whether Covid boosters will be needed every year, as is the case for the rapidly evolving flu virus. But while the virus is so prevalent, it has more opportunities to mutate, creating variants that could make vaccines less effective.

Kingston Mills, professor of experimental immunology at Trinity College Dublin, predicted booster shots would be required "ad infinitum", pointing to how dramatically Omicron has mutated.

"It very much depends on how quickly we control the Sars-Cov-2 virus around the world," he said. "If it's mutating in under-vaccinated parts of Africa, we are at the whim of any of those mutations."

Written by: Hannah Kuchler, Donato Paolo Mancini and Oliver Barnes
© Financial Times