Covid: How we already understand long-term vaccine safety

"We don't know the long-term side effects of Covid vaccines." That's a claim that's still common to see shared online.

But a year is actually considered "long term" when it comes to vaccine safety.

This week marks the anniversary of the first delivery of Covid-19 vaccines under the Covax scheme - as well as being more than 14 months since the first dose was given.

And scientists explain that's more than enough time for all but the rarest side effects to have emerged.

Even though Covid vaccines are relatively new, the processes they trigger in your body are not.

Understanding how they stimulate the immune system can help us understand how quickly we can expect to have any negative reactions.

A tiny fraction of people will have an allergic reaction to the inactive ingredients in the vaccine and this will happen within about 15 minutes of receiving it.

Shortly after receiving the vaccine, the immune system kicks into action.

Your body recognises an alien invader and attacks it with immune cells, the weapons it would use against any virus or bacteria.

We know that any reactions associated with this phase - known as the innate phase - will happen within hours or a couple of days, including the most common side effects, a sore arm, temperature and other mild flu-like symptoms.

A much rarer side effect which has been linked to the mRNA vaccines Pfizer and Moderna - myocarditis, or inflammation of the heart - also occurs in this phase.

Though the exact cause of myocarditis is not understood, we do know inflammation is one of the body's responses to infection or injury.

Vaccine-induced myocarditis is generally mild and gets better on its own or with basic anti-inflammatory drugs such as ibuprofen.

The innate phase kicks off the second bit of your immune response - the adaptive phase, where your body starts to make cells which are specifically tailored to fight off the target virus.

This phase kicks in after about 10 days, which is why it takes the same length of time for the vaccine to begin to have any effect in protecting you against Covid. Your body pumps out new immune cells, in a response that peaks after roughly two weeks and fades away after about 28 days.

A very rare but serious side effect linked to the AstraZeneca vaccine - a specific type of blood clot - happens during this phase and is related to the antibodies produced by your immune system in response to the vaccine. That's why most of these rare clots have been seen within four weeks of vaccination.

Once the adaptive phase dies down - after about a month - you are left with memory cells which afford you protection for months or years after initial exposure - but you're not having any new responses, Dr Victoria Male, a reproductive immunologist in London explains.

So, she says, if you haven't had a reaction after the first couple of months, it is incredibly unlikely anything that happens after that is caused by the vaccine.

There is never a 100% guarantee with anything in medicine, so we can't say it's impossible that something will happen after that time period.

But,"vaccination history assures us that most side effects occur within hours of receiving the vaccine and rare side effects occur within days to weeks", explains Prof Jeffrey Mphahlele, a leading South African infectious diseases researcher.

So we know, from our understanding of how the immune system works gathered over centuries of knowledge, that an individual who hasn't had a reaction to the vaccine in the first couple of months is vanishingly unlikely to have any new side effects after that.

But is it possible that side effects which have already happened are going unnoticed, and may come to light in the coming years?

Countries all over the world have put systems in place to monitor side effects and to share that information with each other.

These systems were successful in picking up both the blood clots and myocarditis which we've already mentioned, despite being extremely rare, with only a handful of cases per million doses.

While milder symptoms - like a sore arm or temperature - are likely to be significantly under-reported, more severe side effects are thoroughly recorded, Dr Male believes.

There are also other major studies into vaccine safety which don't rely on people reporting their own experiences, like the Vaccine Safety Datalink in the US.

Because we expect any side effects to emerge relatively quickly, perhaps a more relevant thing to look at - rather than how much time has passed - is how many doses have been given, suggests Dr Chandrakant Lahariya, an epidemiologist in Delhi.

"Billions of doses have been given, so any side effect not seen yet would be rarer than one in a billion," he said.

Nevertheless, medical systems the world over are still looking for side effects, Dr Lahariya said.

Indeed, while the vaccines have all completed the expected three phases of trials that usually take place before being offered to the general public, they are still being carefully monitored until at least 2023, to make sure even the rarest of events are picked up.

And remember, safety in medicine is all about balancing risks and benefits.

All the evidence suggests the overall risks of catching Covid are many times higher than any risks from the vaccine.