Understanding how vaccines work when you have HIV or cancer

People with weakened immune systems — such as those with untreated HIV infection, people with diabetes or those undergoing chemotherapy for cancer — are far more likely than people with healthy bodily defences to end up in an intensive care unit or die from Covid-19. 

But is it safe for this part of the population to get vaccinated against Covid and how well do jabs work to protect them from falling seriously ill?  

Scientists say the higher your risk for Covid-related complications, the more important it is for you to get vaccinated. That’s why most countries start off their vaccination campaigns for people aged 60 years or older. Studies have shown that the strongest predictor of how ill Covid will make you, is age. According to the US government’s Centres for Disease Control more than 80% of Covid-19 deaths occur in people over age 65, and over 95% in people older than 45. 

Why are older people more at risk? 

They generally have weaker immune systems than younger people and also more illnesses — or comorbidities — that make them vulnerable to ending up on a ventilator or dying of Covid. In South Africa, people older than 40 years make up the bulk of those living with comorbidities (such as high blood pressure, diabetes or cancer), according to a South African Medical Research Council policy brief. 

But what happens if a vulnerable person also needs a jab against another illness, for instance, flu or rabies — can they have the shots simultaneously or do they need to space them out?

Lastly, if you can’t make the appointment for your second Pfizer-BioNTech Covid jab 42 days (the health department’s guideline) after the first one, will the shot still work well? 

We asked the epidemiologist and director of the Centre for the Aids Programme of Research in South Africa, Salim Abdool Karim, for answers in an interview we conducted on Newzroom Afrika. 

Bhekisisa: Is it safe to get vaccinated if you have a weakened immune system because of, for instance, HIV or cancer?

Salim Abdool Karim:

“It’s perfectly safe. And it is critically important that individuals who are immunocompromised get vaccinated — that is, if you have cancer or HIV, or if you’re on chemotherapy, or you have a lymphoma, all of those conditions.

“What happens in those individuals is that if they are very severely immunocompromised, we find that they don't respond well to the first dose of the vaccine. But they tend to respond well when they get a booster dose — so they have a slightly delayed response. 

“And when they are very severely immunocompromised, sometimes they even need a third dose. 

“I would say that every person who is immunocompromised should be scheduling themselves to get vaccinated as soon as possible. They should get fully vaccinated, so with two doses of the Pfizer vaccine or a single dose of the Johnson & Johnson vaccine [these are the two vaccines which SA uses in its national roll-out].” 



The health department says that if you’ve been vaccinated against the flu, you should wait for two weeks before you get a Covid jab. Is that the same for other vaccinations and why do you need to wait? 

“When you give somebody a vaccine, they have an immune response that leads to certain chemicals in the blood being raised. That’s part of what we call an acute phase reaction. 

“The particular chemical I’m referring to is called interferon. When you’ve been vaccinated, your interferon levels go up high and that can interfere with a vaccine response [the response of another, different vaccine given after the first jab]. 

What are interferons?

Interferons tell your immune system when a germ, which needs to be fought off, is in your body. They prompt killer immune cells to fight invaders. In the case of Covid-19, the germ would be the SARS-CoV-2 virus. Interferons are proteins and got their name because they “interfere” with viruses and stop them from replicating.

What is an antigen? 

An antigen is an invader in your body which induces your immune system to produce antibodies against it. In the case of SARS-CoV-2, antigens are proteins on the outer shell of the virus.