The Johnson & Johnson vaccine has followed AstraZeneca with similar reports of rare blood clots, prompting re-evaluation of their effectiveness and associated risk.
While this is worrying for potential recipients, these type of problems go with the territory. Making an effective vaccine and administering it to millions of people is precarious at the best of times. Serious unforeseen side effects can suddenly emerge in large populations and scupper rollout plans, or introduce temporary delays.
Scientific evidence suggests the Johnson & Johnson jab will be subject to some age restrictions in the same way the AstraZeneca vaccine has been curbed in Europe – prompted by "abundance of caution".
There is enough evidence, however, confirming both are highly effective in preventing severe illness and death from Covid-19. For those over 60, the benefits from either vaccine far outweighs the risk of getting the rare clot being reported.
Why is the Johnson & Johnson vaccine being re-evaluated?
The problem has arisen in the US, where the single-dose vaccine was developed. Some 7 million doses have been administered there. Among these, six cases of the blood clot, known as cerebral venous sinus thrombosis (CVST), were reported.
The cases all occurred in women aged 18- 48, with symptoms occurring six to 13 days after vaccination, according to the Food and Drug Administration (FDA), which recommended a pause in use.
This follows the European Medicines Agency (EMA) finding that the AstraZeneca vaccine may be the cause of the same extremely rare disorder. Based on EMA reports, the National Immunisation Advisory Committee (Niac) recommended on Monday that its use in Ireland be restricted to people over 60.
Is the cause of clots the same for both vaccines?
This is the likely scenario as both are “adenovirus” vaccines. That means they use a modified and harmless virus – in this case an adenovirus – as a delivery system for genetic instructions which teach the immune system to produce spike proteins like those that adorn the outer shell of SARS-CoV-2, the virus that causes Covid-19.
When the body begins producing the harmless spike protein, the immune system reacts and learns how to fight the real thing.
These vaccines are easier to manufacture than mRNA-based formulas like the Pfizer and Moderna vaccines and they can be kept at regular refrigerated temperatures.
How reliant is Ireland on the Johnson & Johnson vaccine?
Some regarded it as a gamechanger, others more familiar with the trials and tribulations of new vaccines were more cautious. But it was due to play a big role in Ireland’s vaccination programme.
Ireland’s population entitles it to 605,000 doses of the overall EU total by the end of June. In addition, it was considered useful for hard-to-reach, vulnerable and high-risk people, including those who might need to be vaccinated quickly and not wait for a second dose – as required with other vaccines.
When restrictions to AstraZeneca and uncertainty over the Johnson & Johnson jab are combined, delays in rollout are inevitable – even if the FDA reaches its verdict quickly.
What happens next?
The fact that such a small possible risk has been identified with two different vaccines on separate sides of the Atlantic is reassuring; it shows “pharmacovigilance” systems are working.
The US regulators will try to estimate how many such cases would have been expected by chance, and thus what the risk (if any) of vaccination would be – in much the same way the EMA and Niac did.
Even if all of the latest reported cases were caused by the US vaccine, the risk of less than one in a million would have to be set against the benefits of protection from Covid-19, a disease which causes clotting in many cases.
The problem is the latest “pause” will further undermine public confidence in vaccination at a time when it needs to be scaled up and, ironically, supplies are at last picking up.