Why should you wait for two weeks after a flu jab before getting vaccinated for Covid-19, and is it safe to get a shot if you have a weak immune system? We break it down.

  • If you have untreated HIV or if you’re undergoing chemotherapy, your immune system is likely compromised and you should get a Covid-19 jab as soon as possible.
  • You shouldn’t get a Covid-19 jab within two weeks of a vaccine for a different illness, so that you allow your immune system to respond in the best possible way to each shot.
  • It’s safe to wait for longer than 42 days to get your second Pfizer shot (although it’s not recommended by the health department) – but you shouldn’t wait longer than six months.

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 defenses 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-19 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 of 60 years or older.
Studies have shown that the strongest predictor of how ill Covid-19 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-19. 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 Coid 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.
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-19 jab. Is that the same for other vaccinations and why do you need to wait?
Salim Abdool Karim:
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.
It’s therefore important that we separate out these two vaccines, because we don’t really know how they work together. There are some vaccines that we do give together, they’re perfectly safe to give together, for instance, DTP (diphtheria, tetanus and pertussis/whooping cough) vaccine.
We’ve been giving those to children for decades. But we know that combination is effective, and you get an immune response to each of the three separate antigens that are in the vaccine.
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.
But when you’re dealing with a vaccine, for instance, measles, then you want to create a gap. So if you take [the] flu vaccine today, you want to give a gap because you want your interferon levels to come down and normally that happens within seven to 14 days. Soabout 14 days later, you can take another vaccine.
[This would be the case] whether it’s taking a Covid vaccine or any other vaccine – spacing out vaccines is important in terms of ensuring that each vaccine gets a good response from the body that is not being interfered with by the previous vaccination.
Our guidelines in South Africa say that, in the case of a Pfizer vaccine, you need to wait for 42 days after your first shot before getting a second jab. What happens if you wait for longer – does it make the vaccine work less well?
Salim Abdool Karim:
The original recommendation with the Pfizer vaccine was that you should take the second dose 21 days after the first dose. So the gap between the two doses originally authorised by the Food and Drug Administration [the US government’s medicines regulator] for emergency use, was 21 days.
But there were several studies that showed that when you increase that gap – and it actually doesn’t only apply to the Pfizer vaccine, it’s the case for some other vaccines as well – the immune response from the second dose can be better.
When they increased that gap in the United Kingdom to three months,they found that the immune responses were still very good. In fact, they were better sometimes.
So there’s actually no problem with increasing the gap beyond the 42 days. In fact, your immune response may even be stronger.
We recommend that you shouldn’t delay the second dose for beyond six months after the first dose. So if you can’t do it in the first 42 days, get it done as soon as possible thereafter, but don’t delay more than six months.
*This story was produced by the Bhekisisa Centre for Health Journalism. Sign up for the newsletter.