Why would you experience worse side effects after your second Pfizer shot?

I was delighted when I felt fit as a fiddle after receiving my first Pfizer shot to guard against Covid-19.

Then I started hearing about people who'd gone for their second Pfizer jab. Some felt  hundreds afterwards, but others felt awful and said they'd experienced worse side effects the second time round — sometimes feeling flat for up to two days.

Why might some people experience stronger side effects after their second dose? We asked a series of health experts.

Before seeing what they have to say, it's important to note that Pfizer is a mRNA vaccine. This type of vaccine contains a molecule (mRNA) that teaches your body’s cells how to make the proteins needed to trigger an immune response.

WHAT THE EXPERTS SAY

PROFESSOR VERONICA UECKERMANN

Head of the Infectious Diseases department at the University of Pretoria

The immune system is already primed by the first jab. When you get the second one, your immune system recognises the proteins [produced by the cells that have taken up the vaccine] and mounts an immune response to it.

The side effects that people commonly experience are flu-like symptoms such as body aches, fever, headache — like one would with a viral illness. This is because the immune system is responding as it would to a viral infection.

PROFESSORS HANNELIE MEYER AND ROSE BURNETT

Meyer is the head of the SA Vaccination and Immunisation Centre (Savic) at Sefako Makgatho Health Sciences University. Burnett is a scientific adviser at Savic

When you get the first dose of the vaccine, your immune system is activated and your immune cells are basically “primed” or trained to identify and get rid of SARS-CoV-2 [the virus that causes Covid-19] without actually exposing you to this virus.

This immune response includes an inflammatory response, which is responsible for the side effects. Though not experienced by everybody, these side effects are normal and usually mildtomoderate. They signify that your body is building protection against SARS-CoV-2.

However, such protection is not optimal after the first dose, hence the need for a second.

When you get the second dose of the vaccine, you already have ready-made antibodies and primed T-cells that respond robustly to the spike proteins produced by the cells that have taken up the vaccine, which explains why some people experience a strong reaction.

Similar to the first dose, these side effects should subside within two to three days and should not discourage you from going for the second dose.

While the first dose provides only partial immunity, two weeks after the second dose you will have high protection against severe disease, hospitalisation and death.

Don’t worry though if you don’t have side effects at all. This doesn’t mean that you haven’t mounted an immune response, since many people produce good immunity without experiencing these side effects.

PROFESSOR SHABIR MADHI

Professor of vaccinology, director of the Vaccines & Infectious Diseases Analytics Research Unit at Wits University and co-director of African Leadership in Vaccinology Expertise

It is well recognised that reactogenicity [the capability of a vaccine to cause a reaction] is higher after the second dose of mRNA vaccines than after the first.

This might be because the immune system has been “sensitised” and mounts a more intense response with the second dose. However, the converse is seen for the AstraZeneca vaccine.