How will covid-19 vaccine safety concerns impact vaccine confidence?

Will the UK’s decision to offer an alternative to the Oxford/AstraZeneca covid-19 vaccine for under-30s, following safety signals, impact vaccine up-take, ask Pauline Paterson and Ed Pertwee 

The UK recommendation that under-30s are offered an alternative vaccine to the Oxford/AstraZeneca ChAdOx1 nCoV-19 vaccine (AZD1222), as a precautionary measure following a review of 19 reported deaths and 79 blood clotting cases following vaccination, will likely impact public vaccine confidence and acceptance. [1]

Currently the data are mixed. As part of an ongoing study at the Health Protection Research Unit (HPRU) in Immunisation, we monitor social media conversations around covid-19 vaccines closely. So far the social media signals are relatively positive. We observed a spike in discussions around the Oxford/AstraZeneca covid-19 vaccine when the announcement was made on 7 April 2021, along with an uptick in negative social media sentiment. However, this was less severe than during mid-March when a number of European governments temporarily suspended use of the vaccine altogether. For the most part, social media discussion in the UK since the 7 April announcement has been dominated by vaccine advocates reassuring people that the risks are small. Many members of the public have also taken to social media to point out that the risks from the Oxford/AstraZeneca covid-19 vaccine compare favourably with risks from common drugs, such as blood clot risks from the contraceptive pill. Understandably, many people have questions and concerns, but so far it does not seem as if ideological opponents of vaccination have been able to exploit the situation in the way that some feared. 

However, despite the positive messaging, some polling data presents a less optimistic picture. A nationally representative survey of 2,090 adults in the UK, conducted 9-11 April 2021 by collaborators at ORB INTERNATIONAL, illustrates a drop in confidence in the safety of the AstraZeneca covid-19 vaccine compared with two weeks previously, with a larger drop of confidence seen in women (from 75% to 64%) compared with men (from 78% to 75%) [unpublished data].  

Research we conducted at the Vaccine Confidence Project last Spring with parents of young children already highlighted some concerns of vaccine safety. [2] The recent safety signals and programmatic change may indeed increase concerns for some. The lower risk of severe covid-19 in under 30s with no underlying health conditions, in parallel with the slightly higher incidence of these rare side effects in younger adults, may reduce their willingness to get vaccinated. Others may be reassured by the prompt and comprehensive safety reviews by the Joint Committee on Vaccines and Immunisation (JCVI), MHRA and EMA. [1, 3-4] Also, under 30s may have their safety concerns alleviated by the offer of alternative vaccines that have not had these safety signals.

The occurrence of these blood clots linked to low platelet counts after having the Oxford/AstraZeneca covid-19 vaccine are extremely rare—of the 20 million Oxford/AstraZeneca covid-19 vaccines given, the overall risk of these blood clots is around one in 250,000 people. [3] All medicines and all vaccines carry some risks. And rarer safety signals only emerge when vaccines are given at a large scale. The risk of serious health effects from covid-19 are far higher, differing depending on a number of factors including age and prevalence of covid-19 within the population. [1] Research in psychology has shown that there are strong variations in risk perceptions among individuals, especially with rare events which are difficult to comprehend, and often over-represented in the mind. [5] Fear amplifies risk estimates, and mere reading about these rare events will increase their imaginability and hence also their perceived risk. [6] It is also worth bearing in mind that there is considerable uncertainty in the background risk of Cerebral Venous Sinus Thrombosis (CVST), ranging from between 2 and 16 cases per million people, [4] as well as uncertainty as to the actual incidence rate of these blood clots following immunisation. 

Past research has shown that vaccine scares, legitimate or not, have derailed immunisation programmes. And events occurring in one country can impact confidence in another. The US Food and Drug Administration recently pausing the use of a different vaccine in the USA, while reviewing a number of cases of CVST seen in combination with low levels of blood platelets (thrombocytopenia) this week, will also likely affect the public’s risk perceptions of covid-19 vaccines. 

Vaccination is one of the critical tools we have in controlling this pandemic, along with mask wearing, physical distancing, hand hygiene, testing and self isolation. [7] We have a strong covid-19 vaccination programme in the UK, with over 30 million adults having received a first dose so far. [8] To continue with this success, it will be key to acknowledge uncertainties and safety fears, to continue to build and sustain trust through conversations with the public via trusted messengers, together with prompt and scientifically based programmatic responses. 

Pauline Paterson, assistant professor and co-director of The Vaccine Confidence Project, London School of Hygiene & Tropical Medicine, UK.

Ed Pertwee, research fellow, The Vaccine Confidence Project, London School of Hygiene & Tropical Medicine, UK.

Competing interests: PP is funded by NIHR (for HPRU in Immunisation research), EP is funded by the Cabinet Office (for research on COVID-19 vaccine confidence and infodemics), and both PP and EP are funded by UNICEF (for research on immunisation and COVID-19 in Europe and Central Asia).


1] JCVI. JCVI statement on use of the AstraZeneca COVID-19 vaccine: 7 April 2021. Independent report. Published 7 April 2021. URL available at: Accessed 13 April 2021.

2] Bell S, Clarke R, Mounier-Jack S, Walker JL, Paterson P. Parents’ and guardians’ views on the acceptability of a future COVID-19 vaccine: A multi-methods study in England. Vaccine. 2020; 38:7789–7798.

3] Medicines and Healthcare products Regulatory Agency. MHRA issues new advice, concluding a possible link between COVID-19 Vaccine AstraZeneca and extremely rare, unlikely to occur blood clots. Press release. 7 April 2021. URL available at: Accessed 13 April 2021.

4] European Medicines Agency. Signal assessment report on embolic and thrombotic events (SMQ) with COVID-19 Vaccine (ChAdOx1-S [recombinant]) – COVID-19 Vaccine AstraZeneca (Other viral vaccines). 24 March 2021. EMA/PRAC/157045/2021. Pharmacovigilance Risk Assessment Committee (PRAC). URL available at: Accessed 13 April 2021.

5] Slovic P and Peters E. Risk Perception and Affect. Current directions in psychological science. 2006; 15(6): 322-325.

6] Slovic P. Perception of risk. Science. 1987; 236 (4799), 280-285. DOI: 10.1126/science.3563507

7] Skegg et al. Future scenarios for the COVID-19 pandemic. The Lancet. 2021; 397, 777-778. 

8] NHS. Announcement regarding AstraZeneca vaccine and next steps. Letter. 7 April 2021. URL available at: Accessed 15 April 2021.