The current resurgence of Covid-19 will prompt many vaccinated people to wonder if they are sufficiently protected against the virus.
Many will have been vaccinated before the summer and are now coming up to the six-month interval most frequently mentioned in the many reports of declining vaccine effectiveness.
It is difficult to be specific about the level of protection an individual will have, due to the number of variables – time elapsed, vaccine used, underlying health status, behaviour and so on.
However, it is clear the effectiveness of vaccines against Covid-19 appears to wear off over time – just as it does for vaccines against measles, mumps and rubella.
The speed at which this happens is only now emerging from rolling studies from around the world. But in any case, wider trends in Covid-19 cases, particularly in countries that immunised their populations early, such as Israel and the UK, point to a loss in effectiveness.
Generally, these studies measure changes in the levels of neutralising antibodies, the immune system’s first line of defence against infection, rather than vaccine effectiveness per se.
And while these levels of vaccine-induced immunity have been falling over time, this isn’t necessarily a bad thing. This is because the body is at the same time creating B and T cells – part of the body’s immune response system – to provide a longer-term response to the threat posed by the virus.
One study found robust B cell protection six months after vaccination; the hope is that this kind of protection will increase over time.
This explains why the vaccines are working better at protecting us from serious illness and death than at simply providing protection against infection.
Vaccines are clearly not providing long-lasting protection against infection with the delta variant; otherwise, highly-vaccinated Ireland would not be experiencing its current wave of cases. One study from Israel found immunity against delta waned in all age groups "a few months" after the second dose of Pfizer was administered.
Another Israeli study found the fall-off in antibodies was most marked among men, over-65s and the immunosuppressed. It also suggested immunity lasts longer among vaccinated people who were previously infected with Covid-19.
A study from Qatar found the waning of protection against infection accelerating after four months.
But it also pointed to the role of behaviour in this decline, with vaccinated people likely to have more social contacts and to adhere less to public health measures.
However, vaccines are faring much better in protecting people from serious outcomes. According to the National Immunisation Advisory Committee (Niac), high levels of vaccine effectiveness against hospitalisation, severe disease and death have been sustained "for at least five to six months".
While acknowledging “some increase” in hospitalisation and severe illness, Niac says, overall, “good” protection against severe disease is sustained.
This is borne out by data from the US, which shows hospitalisation rates among the unvaccinated far higher than among vaccinated people. The multiple varied across different time periods and age groups; overall hospitalisation rates were at least 10 times higher in unvaccinated people as delta became dominant last summer.
The performance of different vaccines varies; one large British study found AstraZeneca provided only 37 per cent protection against symptomatic disease after 20 weeks, half the level achieved by Pfizer.
So far in Ireland, 117 breakthrough infections have been recorded among fully vaccinated people who contracted Covid-19 and had to be admitted to ICU.
This group appear to conform to the research findings elsewhere: overwhelming male and with underlying conditions, and with a median age of 65 years. Only four of these patients did not have an underlying condition.
Sadly, Ireland does not publish data in relation to the hospitalisation or admission to ICU of fully vaccinated people according to the specific vaccine used.
Meanwhile, a study published in the Lancet last week confirmed what many experts had begun to suspect – that being vaccinated does not stop a person transmitting Covid-19.
It found a person with two vaccine doses could still pass on the infection to vaccinated and unvaccinated household members. Vaccinated contacts were less likely to test positive, and they cleared the infection more quickly, but they had a similar peak viral load to unvaccinated cases.
Fully vaccinated people have a “lower, but still appreciable” risk of becoming infected with the delta variant, the authors concluded, and as a result, vaccination alone will not be sufficient to stop the virus spreading.
In summary, therefore, the data points to vaccines still providing good protection against illness for most healthy individuals. More vulnerable groups need boosters, but these are already being provided. Finally, as Israel’s experience shows, boosters can help control fresh waves of Covid-19 – which is why so many governments are so keen to widen access to them.