Dr Kevin McCarroll
Medical Matters
We are now nearly 15 months into a pandemic that has claimed the lives of nearly four million people globally and resulted in about 5,000 deaths in Ireland. However, we are now at a juncture where the vaccination programme is well under way and hospitalised cases of Covid are at their lowest in nine months. At present, over three million vaccine doses have been administered with about a third of the population now fully immunised. Despite this, concern has arisen regarding the impact of the recently identified ‘delta strain’ of the virus which has the potential to threaten the planned unwinding of restrictions this summer.
After two doses of the Pfizer vaccine, there is 88% protection against developing symptomatic Covid though with AstraZeneca this figure is 60%”
But are these concerns warranted? The delta strain emerging from India is now one of the four main coronavirus variants globally. Unfortunately, it is estimated to be about 50% more transmissible than the UK Kent or “alpha variant’ which resulted in the second wave of infections earlier in the year. It is also estimated to have a twofold greater risk of hospitalisation. In the UK, there has been a rapid rise in this new variant where it now accounts for the vast majority of cases, as it now does in some other EU countries including Portugal. In Northern Ireland too, it is estimated to comprise about 50% of infections. While most cases of Covid are now affecting younger age groups this also increases the potential for spread to older adults, many of whom are not fully vaccinated.
So what protection does vaccination offer against this new variant? After two doses of the Pfizer vaccine, there is 88% protection against developing symptomatic Covid though with AstraZeneca this figure is 60%. However, a more important figure is the protection against hospitalisation due to Covid, which for both vaccines is very reassuring at about 94%. Despite this, many adults have only received one jab of either vaccine including about 70% of adults aged between 60-69. Studies suggest that one dose offers only about 35% protection against symptomatic Covid, though with Pzifer still preventing hospitalisation in 94% compared to 71% with AstraZeneca. This means that a significant proportion of adults still waiting on their second jab are at risk of symptomatic Covid and/or more severe disease. In fact, only about a third of eligible adults are estimated to be fully vaccinated. While the delta variant still accounts for a small proportion of Irish cases, there has been a number of outbreaks and it could yet become the dominant strain.
Pfizer and Moderna’s mRNA vaccines are more effective at inducing neutralising antibodies than the adenovirus-vectored vaccines from AstraZeneca and Johnson & Johnson. There is also the phenomenon of ‘antivector immunity’ where antibodies produced against the viral vector used to deliver the main vaccine component may reduce the effect of a second or booster dose. This is important as already plans are being made for booster vaccines to be given next year. One option to get around this is consider using a different booster vaccine or ‘heterologous vaccination’. A study this year showed a much greater antibody response when Pfizer was administered post AstraZeneca than when using two jabs of the latter. Mixing other vaccines has also been done successfully in the past.
What about using antigen testing in the fight against Covid? There is no question that PCR testing for Covid is the most sensitive at identifying the virus. However, it also picks up very low levels of viral RNA in a person at the early or later stages of the disease who may not necessarily be infectious.
But how reliable is a negative antigen test in ruling out a person who is infectious? A HSE study on rapid antigen testing using nasal swabs found that it is was positive in 80% of people who were strongly positive on PCR. So while it may identify the majority of those who are infectious, it could still miss up to one in five. However, depending on the test used, the ‘pick up’ rate and proportion of false positives is variable. Concerns have also arisen regarding the accuracy with self-testing and lack of standardisation. Despite this, antigen testing is quick with the results within as little as 15 minutes and can be used as a screen in those with no symptoms or contact with Covid. Negative results indicate a low risk of being infectious and therefore should have a role as an adjunct to PCR testing, particularly with gatherings of people in lower risk situations.
Fortunately, in the event of current vaccines being less efficacious as a result of new variants, new mRNA ones can be adapted within months to ensure ongoing protection”
But how long will immunity last? For those who have been infected with Covid, evidence suggests that there is good immunity for at least nine months. With vaccination, the immune response is expected to decline in a significant proportion by one year. For this reason, the government has already done deals to ensure supply of booster vaccines in 2022. Fortunately, in the event of current vaccines being less efficacious as a result of new variants, new mRNA ones can be adapted within months to ensure ongoing protection.