Explainer | Second boosters: what the studies tell us about the fourth Covid-19 jab
- Many countries are offering fourth doses of the vaccines, but most are for at-risk groups like the elderly
- Questions remain over how long the protection will last and if another shot will benefit the wider population
But most of these second booster programmes are just for vulnerable groups like the elderly and the immunocompromised as questions remain over how lasting the immunity is, and if a second jab will benefit the wider population.
There have been some studies done on the effectiveness of a fourth dose but more research is needed. Here is what the data tells us so far.
How effective is a fourth dose for the elderly?
Most of the data so far comes from Israel – the first country to start giving fourth jabs to people aged 60 and over, as well as high-risk groups such as the immunosuppressed and their carers.
One study compared the effectiveness of third and fourth doses of the Pfizer mRNA shot. To do this, it looked at the health records of more than 182,000 people aged 60 and over from January 3 to February 18 – when the Omicron variant was dominant.
In findings published last month in the New England Journal of Medicine, the fourth dose was found to have provided significantly more protection against severe disease, hospitalisation and death for at least a month.
Seven to 30 days after the fourth jab was given – compared to someone given a third shot – the relative effectiveness was 45 per cent against infection, 55 per cent against symptomatic disease, 68 per cent against hospitalisation, 62 per cent against severe disease and 74 per cent against death.