A ground of people. A magnifying glass shows viruses in the crowd

Can I shed a virus after I get a vaccine?

Concerns about shedding coronavirus when out and about have been prevalent over the last year, but what about after receiving a vaccine?

Some shops in the NSW town of Mullumbimby recently puts signs in their windows that read: “If you have had the COVID-19 vaccine we ask you not to enter for two weeks or longer until any symptoms subside.”

The concern was that people who had recently been vaccinated, which is commonly followed by flu-like symptoms, were shedding live virus and were infectious to others. NSW’s north isn’t the only location with such concerns, and restrictions.

So, are vaccinated people shedding virus and putting the community at risk?

Short answer: no. But this idea may have been born from scenarios where it is possible – it just isn’t the case with our current coronavirus vaccines.

Can you shed virus following a vaccination?

Technically yes, but it is extremely rare and only possible with certain types of vaccines.

The oral polio vaccine (OPV) contains a weakened version of the poliovirus and, in some cases, really does revert to its virulent form.

This happens because the weak strain of polio in OPV replicates a little in the gut following vaccination, which gives the body time to recognise the virus and build up defence.

However, this means that if the replicating virus is excreted – pooped out – during this time, it has a chance of remaining in the excrement and spreading upon contact. This becomes a particular problem in regions with poor sanitation and vaccine coverage, which are usually in low socioeconomic areas.

Once it is in this virulent form, the OPV does indeed shed.

This takes a long time though, and that weak virus needs around 12 months to change to a virulent form that can affect an under-immunised community. It’s mostly a problem for these communities because the virus can circulate for longer, giving it extra time to revert, instead of being nipped in the bud.

In 2017, WHO reported that 20 billion doses of OPV had been administered to 3 billion children worldwide and prevented 13 million cases of polio. Of these administrations, the vaccine-derived polio cases numbered less than 760.

If you need a visual, that would be one grain of rice per 79kg, if each grain was a person who received a vaccine.


History of ideas: vaccines and immunisation


Still, this is possibly the reason behind the idea that shedding can occur following the administration of a vaccine: it was born from truth, but the circumstances are different and the extra information to assuage these fears may not have been easily accessible.

However, the difference between OPV and the current coronavirus vaccines is that the latter do not use live virus at all.

What is the difference between a live and inactive viral vaccine?

A ‘live’ virus is one that has the ability to replicate – make copies of itself – when in a host. Technically viruses aren’t alive, but we call them live in a similar way to how we say a website has ‘gone live’.


Why are viruses considered non-living?


Regardless, the whole purpose and goal of a virus is to make as many copies of itself as it can by invading a cell and hijacking normal cellular processes to meet its objective – this is what we call a live virus, and it can be infectious. It will do its very best to find more cells in which to replicate, and shed from one person in the hope of finding another.

The crucial thing here to remember is that a virus, including one in a vaccine such as OPV, can only mutate and/or shed if it is able to replicate.

On the other hand, an inactive viral particle doesn’t have the ability to replicate, and therefore can’t shed. These come in many different forms.

They could be vaccines that contain:

  • An inactivated, ‘dead’ virus that has been killed by heat or chemical treatment. It can’t replicate, because it is dead. This is what flu vaccines use.
  • A viral vector that has been engineered to prevent replication, containing a little bit of virus code but not enough to actually make more virus. This is what AstraZeneca‘s COVID vaccine uses.
  • An RNA fragment of the virus that makes a viral protein. The body can use the RNA blueprint to make a single protein and learn what it looks like, but there isn’t enough RNA to make all the proteins needed to be a proper virus. This is what the Moderna and Pfizer COVID vaccines use. 

Assessing the various vaccine technologies


Now for some fun analogies! If the virus was a dandelion, with all its fluffy, white-topped seeds, a weakened live-virus vaccine would be a sad dandelion. It can still spread, but with difficulty – it doesn’t catch the air very well.

The flu virus would be a squashed, dried dandelion that was soaked in ethanol. We can look at it and recognise it, but no seeds can be spread.

A viral vector would be the white fluff – without the seed – encased in resin. In fact, it might not even be the fluff – it could be a leaf or a yellow petal.

An mRNA vaccine would be a blueprint of the shell around the seed.

None of the last three have any way of being blown away and hitting somebody else. Ironically, our wish is granted because we can’t blow those dandelion seeds away.

Because none of the coronavirus vaccines contain live virus, they cannot replicate themselves and they cannot shed.

That doesn’t mean you can’t still show COVID-like symptoms following a vaccine shot – it just means you aren’t contagious.

Are you shedding the virus if you have vaccine side effects?

Another reason why it’s easy to think somebody is shedding following a vaccine is because, for a couple of days, they might look quite similar to somebody who is infectious.

It isn’t uncommon to have COVID-like side effects following vaccination – that means your immune system is working.

When our body is invaded by a virus for the first time, white blood cells rush over and start trying to fight it, but this can cause inflammation as the white-blood-cell army rushes in.

Some of their main attacks include trying to burn the virus out (fever) and trying to cough/sneeze dead or infected cells out of our bodies in mucous. This battle uses lots of resources and energy, so our bodies can feel fatigued and achy, as the blood cells ‘borrow’ proteins for muscles.

Once the battle has been won, the body’s T cells remember how the fight played out, so that the next time they see a virus there is no trial and error and they can just hit it hard.


What are T cells and how do they help immunity?


This is why we often have symptoms following a vaccine. But it’s the result of our immune system trying to come up with the best battle plan, not because of live virus directly hurting the body.

Of course, that means we can still have an immune response without live virus – after all, that’s the whole point – and not shed at all, because your body is responding to something that looks the same but doesn’t behave the same.

After all, some people have flu-like symptoms when they have an allergic reaction, but they aren’t contagious. They are just having an immune response.

Not all responses are the same because we are individual organisms with different circumstances. So some people won’t have any symptoms and others might need a couple of days off work or school – despite all of them receiving the same vaccine.

It means that getting vaccinated against COVID-19 doesn’t put your community at risk. Instead, it’s at greater risk when there aren’t enough people vaccinated, so it’s still critically important to get your jab.

It’s easy to feel drowned in all the available information, but you aren’t alone. The best places to find more information are your local medical centre or GP, and government health organisations and websites.

As immunisation expert Professor Margie Danchin, group leader of the vaccine uptake group at the Murdoch Children’s Research Institute, Melbourne, told Cosmos in May: “People have every right to have questions about these vaccines, and they should have questions.

“The best way for people to have their concerns addressed is to speak with their healthcare provider.”