The potential long-term use of boosters against Covid-19 has been called into question after a study of Israel’s fourth shot for healthcare workers found it was less effective against Omicron than other coronavirus strains.
A fourth shot of Pfizer was given to 154 healthcare workers at Sheba Medical Centre in Tel Aviv, Israel’s largest hospital, to measure the efficacy of vaccines and compare antibody levels.
A further 120 volunteers who had already received three doses of the Pfizer-BioNTech vaccine were given a Moderna shot.
Results showed both vaccines were safe and produced substantial antibodies, but were only partially effective in defending against Omicron.
“Despite a significant increase in antibodies after the fourth vaccine, this protection is only partially effective against the Omicron strain, which is relatively resistant to the vaccine,” said Prof Gili Regev-Yochay, head of infection and prevention and control units at Sheba.
“The rise in antibody levels that we saw with both Moderna and Pfizer is slightly higher than what we saw after the third booster vaccine.
“However, as we have seen within the realm of our ongoing serological studies among Sheba personnel and the rising numbers of personnel infected with Omicron, despite increased antibody levels, the fourth vaccine only offers a partial defence.
“Therefore, the Pfizer and Moderna vaccines, which were more effective against previous variants, offer less protection versus Omicron.”
Antibodies in those who received a fourth shot were compared with a control group of about 6,000 health workers tracked by the hospital since December 2020.
Researchers at the centre said while a fourth shot did not provide optimal protection against Omicron, it did top up antibodies, compared to a third dose of a vaccine and should be encouraged in the elderly and patients with impaired immune systems.
The trial showed two weeks after a fourth Pfizer vaccine dose, antibody levels continued to rise slightly for seven days afterwards.
Sheba Medical Centre has experienced a surge in cases in recent weeks, and has 131 patients admitted as a result of Covid-19.
They include five cancer patients, 10 children, 11 pregnant women and five people in psychiatric care.
In total, 30 are in a serious condition, and four in the hospital’s critical care unit.
Concerns have been raised over the impact of continuous booster campaigns on the global supply chain.
The European Medicines Agency gave a warning that there was little evidence to support a fourth booster campaign, and repeated doses could actually weaken natural immune systems.
An expert group inside the World Health Organisation also advised against repeated boosters in developed countries.
The WHO Technical Advisory Group on Covid-19 Vaccine Composition said vaccine top-ups were not a sustainable global strategy for defeating the pandemic and that new vaccines to deal with emerging variants would be needed.
“Covid-19 vaccines that have high impact on prevention of infection and transmission, in addition to the prevention of severe disease and death, are needed and should be developed,” the group said in a statement on the World Health Organisation website.
Meanwhile, White House chief health adviser Dr Anthony Fauci backed more research into how to develop new vaccines against emerging variants.
“A vaccine that prevents widening systemic disease is what science is working towards,” Dr Fauci told the World Economic Forum on Tuesday, in a talk on how the world will emerge from Covid-19.
“We should have had a more global response to this pandemic. We cannot have a virus circulating freely in one part of the world.
“As long as that happens you will get a virus that impacts all of us.
“If there is a global production capacity for vaccines, that is where we should be heading.”