Coronavirus, Janssen vaccine situation in US and Europe

This article was published on
April 13, 2021

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Expert reaction to Janssen COVID-19 vaccine situation in US and Europe (CDC and FDA recommending a pause in the US while they review six cases of cerebral venous sinus thrombosis (CVST) in combination with low levels of blood platelets (thrombocytopenia); and Johnson & Johnson deciding to proactively delay the rollout of the vaccine in Europe)

Expert reaction to Janssen COVID-19 vaccine situation in US and Europe (CDC and FDA recommending a pause in the US while they review six cases of cerebral venous sinus thrombosis (CVST) in combination with low levels of blood platelets (thrombocytopenia); and Johnson & Johnson deciding to proactively delay the rollout of the vaccine in Europe)


What our experts say

Context and background


Joint CDC and FDA statement:

FDA Twitter feed:

FDA and CDC press conference:

Johnson & Johnson statement:

Media briefing

Media Release

Expert Comments: 

Professor Eleanor Riley

Whilst a causal link between certain COVID-19 vaccinations, platelet abnormalities and blood clots has not, so far, been confirmed, the index of suspicion is rising that these rare cases may be triggered by the adenovirus component of the AstraZeneca and J&J vaccines. Whilst more data need to be collected, and the implications carefully considered, it remains the case that for the vast majority of adults in Europe and the USA the risks associated with contracting COVID-19 far, far outweigh any risk of being vaccinated. Moreover, increasing awareness of the possibility of such side effects means that they should be diagnosed more quickly and treated more successfully.

Professor Anthony Harnden

Although the UK has ordered 30 million doses of the Janssen vaccine it has not yet been approved by our regulator the MHRA. The Janssen vaccine uses the same technology platform as the Oxford Astra Zeneca vaccine, albeit a different adenovirus vector. So although the most recent data about this rare adverse event in the US still has many uncertainties it clearly requires both the MHRA and JCVI to scrutinise any new data related to both vaccines as it emerges. The observation of cases of thrombosis and thrombocytopenia in those receiving the Janssen vaccine in the US will need to be carefully reviewed – depending on outcomes of any review there may be implications for the recommendation of the Janssen vaccine in the younger age groups in the UK where the risk from severe Covid is much less than in older age groups and in those with underlying illnesses

Professor Ian Douglas

From the limited information available right now, these 6 cases with the J&J vaccine appear to be similar to those seen with the AZ vaccine.  Given the heightened awareness of this issue with COVID vaccines, it’s not surprising that we see more cases being reported as this snowball effect is a well known feature of safety reporting with any medicine.  Whilst it’s too early to conclude whether they are causally related to the vaccine, this type of stimulated reporting is likely to help regulators better determine what’s related to the vaccine and what isn’t.  If a causal link is established, it’s also possible the reports will help us establish possible risk factors for experiencing this outcome, and ultimately help find ways to minimise any risk.

As we’ve seen, each country makes its own decision about COVID vaccines based on their known risks and benefits, but also other factors such as the availability of other vaccines, circulating virus levels, local vaccine confidence and political pressures.

 It’s vital to stress all indications are that this is incredibly rare, in this instance around a 1 in a million chance based on what we know to date about the J&J vaccine. To put this into perspective, it’s similar to the chance of being struck by lightning in any given year in the UK. On the other hand, the risks from COVID-19 are substantial. If all 6.8 million people who’ve received the J&J vaccine in the US were infected with the virus, several thousand would likely die and many more, including younger adults, would experience serious and long-lasting after effects

Professor Adam Finn

The announcement of the recommended suspension of the one dose Janssen COVID vaccine by the authorities in the USA following 6 cases of cerebral venous sinus thrombosis with low platelets among nearly 7 million people given the vaccine echoes recent events in Europe around the Oxford-AstraZeneca vaccine.  These two vaccines are similar in the way they work, using two different adenovirus vectors to carry the coronavirus spike protein gene into the recipient’s arm.  The numbers of cases are, once again, very small but the combination of severe thrombosis with thrombocytopenia is characteristic of the cases that have been reported previously.  Although it’s too soon to draw any firm conclusions, this development does raise the possibility that at least some adenovirus vectors either of themselves or in combination with the SARS CoV2 S protein gene can cause this idiosyncratic reaction in a very small proportion of individuals.  This may help give us a clue towards understanding the mechanism or a way to prevent this problem from occurring.  Given the importance of these vaccines for the timely control of the pandemic, investigation of this phenomenon is now an extremely urgent international priority.

Dr Peter English

This would appear to be a highly precautionary move by the US’ agencies.

According to reports to date, there appear to have been six cases of central venous sinus thrombosis (CVST), in over 6.8 million doses of vaccine given.

The alert related to the AstraZeneca vaccine, where a similar association is considered possible, will have raised awareness of a possible association with other vaccines.  Nevertheless, the fact that such a small possible risk has been identified is very reassuring – it shows that pharmacovigilance systems are working.

The regulators will be trying to estimate how many such cases would have been expected by chance, and thus what the risk (if any) of vaccination would be.

Even if all of the cases were caused by the vaccine, the risk of less than one in a million would have to be set against the benefits of protection from Covid-19 disease; a disease which, in itself, causes clotting in many cases.

The call for a pause may dent confidence in vaccination.  We must hope that, if it does, this will be temporary; and that the rapid action and openness will inspire confidence that vaccine safety is taken extremely seriously.


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