Covid-19 boosters are firmly on the political agenda, despite a lack of consensus among scientists about the benefit they confer.
The main reason stems from indications that vaccines are not providing lasting protection against infection.
Waning protection may be behind the surge in cases this summer in the UK and Israel, the two countries that were out of the traps first in vaccinating their populations at the start of the year. (Though the fact that they did not complete the job by vaccinating a high proportion of people, as Ireland has, did not help.)
The most comprehensive data we have so far, from Public Health England, points to a decline in protection after about five months among older people.
So far, there is little evidence of waning immunity among under-65s, it found, but this could change with time.
And protection against hospitalisation remains strong, particularly for the Pfizer vaccine. In other words, vaccines are still the "life jacket" against severe illness, about which the World Health Organisation's (WHO) Mike Ryan has spoken.
Other research has shown a more alarming decline in protection, such as a US study that said the protection offered by Pfizer and Moderna vaccines fell from 74 per cent in the spring to 53 per cent in summer. But this study included infections with no symptoms in the analysis and did not look at the impact of vaccine effectiveness on serious illness.
A second reason why governments are keen to push out boosters is because they may reduce the onward transmission of the disease. That should mean fewer cases and, of relevance to Ireland, less pressure on a stretched health service in winter.
Early results from Israel appear to sustain this reasoning. There, daily infections have fallen more than 80 per cent since peaking in early September, with severe cases nearly halved.
Among over-60s – the first group to receive boosters – infections began declining rapidly about two weeks after third doses were administered, while still climbing among other age groups. However, Israel has far fewer people vaccinated than Ireland and its success is also partly due to strict enforcement of Covid passes and rules on masking.
For the WHO, boosters remain, to use Dr Ryan’s analogy, a superfluous “second life jacket”.
In its most recent policy statement earlier this month, the WHO said the evidence for boosters “remains limited and still inconclusive on any widespread need for booster doses following a primary vaccination series”.
Just because antibodies decline in the months after vaccination doesn’t mean a person is less protected, the WHO notes, as cells have longer-term ways of generating immunity.
Critics argue that in a pandemic you often have to act without waiting for all the evidence to come in conclusively. For example, the initial caution of the WHO and Irish public health officials on masks in spring 2020 hasn’t stood the test of time.
What about the developing world?
While the issue has divided scientists globally and in Ireland, there is consensus on the ethical dimension. Global vaccination is seen as the way out of the pandemic and, for now, the greatest need for vaccines is in the developing world, where supplies are lacking. A third dose of vaccine in an Irish arm is a first dose denied to someone in the developing world.
There therefore needs to be a compelling scientific case for providing boosters before the green light is given to their administration more widely in the Irish population.
They are already being provided to the very old and the immunocompromised, and most people in nursing homes, on the basis of risk. A case could also be made for over-60s who were given AstraZeneca, the vaccine most subject to waning protection.
The US and the UK have ploughed ahead with the approval of boosters, just as they were quick to give initial authorisation to Covid-19 vaccines, but other European countries are proceeding more cautiously.
Germany has approved boosters for over-60s and workers in high-risk areas, while France is giving boosters to over-65s. Sweden is waiting until 2022 before giving booster shots generally and Switzerland is stockpiling supplies for next year.
Extending availability to boosters won’t solve the immediate problem of rising virus cases, but it might help in a few months’ time.
It also suggests we will have to keep giving boosters at regular intervals, until the pandemic is over.