Covid vaccine side-effects: what are they, who gets them and why?
What are the most common side-effects from the Covid vaccines?
According to Public Health England, most side-effects from the Covid vaccines – Pfizer/BioNTech and Oxford/ AstraZeneca – are mild and short-lived. These include soreness where the jab was given, feeling tired or achy and headaches. Uncommon side-effects include having swollen lymph nodes.
Early reports that some people had severe allergic reactions, particularly to the Pfizer jab, led the UK’s Medicines and Healthcare products Regulatory Agency (MHRA) to offer new recommendations, including that anyone receiving the Pfizer jab be monitored for 15 minutes after the event.
Are blood clots a side-effect of the vaccines?
There have been some cases of blood clots or low platelets among recipients of both Oxford and Pfizer vaccines, but Dr Phil Bryan, vaccines safety lead at the MHRA, said this was to be expected. “These are not uncommon events, we expect some of these to happen,” he said. “The rates of venous thromboembolism are not occurring at greater frequency after either vaccine than the rates in an unvaccinated population.”
However, the MHRA and other bodies are investigating reports of a rare type of blood clot on the brain called cerebral venous sinus thrombosis (CVST). On Thursday the MHRA said there had been five cases of CVST occurring with lowered platelets among men aged between 19 and 59 out of 11 million people who had received the Oxford jab. One of the men is known to have died.
“What we don’t know is whether these cases have been caused by the vaccine,” said Bryan. “We are working closely with our experts, our haematologists, to try to gather more information to determine this because these are illnesses that do, very rarely, also happen naturally.”
Bryan said the Oxford and Pfizer vaccines were highly effective in preventing symptomatic Covid infections, hospitalisations and deaths. “Where we are now is that no causal association with what is still an extremely rare medical event has been proven for the Oxford University/AstraZeneca vaccine,” he said. “What we do know is that these are highly effective vaccines and we still have a huge burden of Covid disease in the population. So right now the balance of benefits and known risks of the vaccine are favourable.”
More than a dozen EU countries paused their use of the Oxford jab “out of an abundance of caution” following rare blood clot events, but a number of official bodies including the World Health Organization have said it is important the vaccine continues to be used given the benefits it offers against Covid.
That position was given a further boost on Thursday when the European Medicines Agency (EMA) released the results of a review concluding that the Oxford vaccine was not associated with an increase in the overall risk of blood clots. It said that while a link to the rare and unusual cases could not definitively be ruled out, and a warning would be added to the product, the benefits outweighed the risks.
“I want to reiterate that our scientific position is that this vaccine is a safe and effective option to protect citizens against Covid-19,” said Emer Cooke, EMA executive director.
Why do the common side-effects occur?
“The sore arm can be either due to the trauma of the needle in the muscle, or local inflammation in the muscle probably because of the chemicals in the injection,” said Prof Robert Read, head of clinical and experimental sciences within medicine at the University of Southampton and director of the NIHR Southampton Biomedical Research Centre.
“The other common side-effects – the muscle aches, flu-like illness and fatigue – are probably due to generalised activation of the immune system caused by the vaccine. What this means is that the white blood cells that are stimulated by the vaccine to make antibodies themselves have to secrete chemicals called cytokines, interferons and chemokines, which function to send messages from cell to cell to become activated.”
Read said that for some people the process is without symptoms, but for others it generates these common sideeffects.
Are the common symptoms worse for the first or second shot?
For the Oxford jab, it seems such side-effects are more common after the first shot. “When compared with the first dose, adverse reactions reported after the second dose were milder and reported less frequently,” the MHRA notes.
However, for the Pfizer jab the reverse appears to be true. “Side-effects such as fever, chills, tiredness and headache throughout the body were more common after the second dose of the vaccine,” the US Centers for Disease Control said.
Read said: “We are a little baffled about this, but it may be due to the fact that the Oxford University/ AstraZeneca vaccine has an adenovirus vector, which stimulates the immune system strongly in the first dose and less strongly in the second.”
Are people who have had Covid more likely to get these side-effects?
Some data suggests this may be the case. According the ZOE Covid symptom study released last month and
looking only at the Pfizer jab, about a third of vaccine recipients who had previously had Covid reported having a whole-body side-effect (such as chills), compared with 19% of those who had not had Covid.
Read said: “Usually if you have experienced a natural infection – in this case with the coronavirus – and are then challenged with something that looks quite similar – in this case the vaccine – an efficient immune system responds very quickly to the second challenge.”
Are there other groups who are more likely to experience side-effects?
The data suggests that side-effects are more common among younger recipients. “We have seen that older people are getting much milder side-effects. It’s not obvious why this should be – perhaps younger people have a much more robust reaction to the priming by the first dose,” said Read.
Does a stronger set of side-effects suggest you will have better protection against Covid?
Not necessarily. “All the evidence we have is that there isn’t a correlation,” said Simon Kroll, professor of paediatrics and molecular infectious diseases at Imperial College London. “That evidence best comes from looking at the responses from older people and younger people because the evidence is that the vaccines are very effective right across the age range, but the side-effect profile is weighted towards younger people.”