Hybrid immunity offers the greatest protection against severe disease from Covid — and previous infection provides higher protection than vaccination alone, a review has found.
However, all sources of protection waned “within months” against reinfection, the analysis found.
The term hybrid is used to describe the immunity provided by a combination of infection and vaccination.
They wanted to know how strong the level of protection in people with hybrid immunity was, compared to those who had been infected without being vaccinated.
“There are gaps in the literature on the magnitude and duration of protection conferred by previous infection, both among individuals who have not been vaccinated (ie, the effectiveness of previous infection alone) and among individuals who have been vaccinated (ie, the effectiveness of hybrid immunity),” said the paper, which has been published in The Lancet Infectious Diseases.
Previous reviews had estimated the durability of protection conferred by previous infection.
But none had looked into the durability of protection conferred by hybrid immunity or compared the durability of the different types of protection, said the researchers.
So they set about comparing data from 26 studies.
The analysis found people with previous infection alone, and those with previous vaccination only had “rapidly waning protection” against infection with the Omicron variant.
“Previous infection was found to provide higher protection against reinfection and more sustained protection against hospital admission or severe disease than vaccination alone,” said the study.
However, people with hybrid immunity had the most robust and longest protection of all, “emphasising the importance of providing vaccination to previously infected individuals,” they said.
The analysis shows protection against admission to hospital remains high 12 months after developing hybrid immunity.
Anyone who developed hybrid immunity in the period had a 95 per cent lower chance of suffering a serious case of the disease, according to the paper.
Those who were infected a year before, but were not vaccinated, had a 75 per cent lower chance of developing a severe disease and requiring admission to hospital.
Protection against reinfection was lower, at 42 per cent for those with hybrid immunity, compared to 25 per cent for those who had been infected before but were not vaccinated.
The researchers said protection provided by previous infection “should not detract from the need for vaccination” because infection-induced protection against reinfection wanes rapidly, and vaccination increases durability.
“Furthermore, there are serious risks associated with infection,” said the paper.
“These include the risks of hospital admission, ICU admission and mechanical ventilation, and death. They also include the risk of developing post-Covid-19 complications.
“Additionally, those who recover from severe Covid-19 have a higher risk of cardiovascular complications, neurological complications, dementia, diabetes, and chronic respiratory problems.
“Vaccination is therefore a safe intervention to avert severe disease outcomes and to reduce post-Covid complications.”
A recent large-scale global analysis found pregnant women with Covid are seven times more likely to die and face a substantially higher risk of suffering from serious complications.
Expectant mothers infected by Covid-19 are more than three times more likely to be admitted to intensive care and 15 times more likely to require a ventilator to help them breathe.
They face a 23 times higher risk of developing pneumonia and are five times more likely to suffer from blood clots.
The study also suggests having Covid during pregnancy also increases the risk that the baby will need to be admitted to intensive care, said the study.
Researchers said the risks underscore the need for pregnant women to be vaccinated against the virus.