How are COVID-19 vaccines faring over time? Here's what we know about waning immunity
Around the world, COVID-19 vaccines are working remarkably well to reduce rates of severe disease and death.
But a growing number of studies suggest their ability to protect people from milder illness and asymptomatic infections is waning with time. But by how much?
It's difficult to "put a hard number on it", says vaccine researcher Kylie Quinn.
"We've moved from these nice, tidy randomised control trials … [to] having to measure it on the run," said Dr Quinn of RMIT University.
For researchers, it's been tricky to tease out how much of what they're seeing is the result of waning immunity, and how much comes down to the Delta variant, the relaxing of restrictions, and the vulnerability of people vaccinated early on.
"I think it's clear that Delta is more of a challenge for these vaccines," Dr Quinn said.
"But now we're starting to get cleaner data sets that show there is an additional challenge of waning immunity."
How much does this matter? And what does it mean as Australia starts to open up?
Let's take a look.
Evidence shows protection against getting COVID wanes with time
To investigate how well COVID-19 vaccines are working to protect against infection, UK researchers recently assessed more than 350,000 COVID-19 test results taken as part of a large household survey between May and August this year, when the Delta strain was dominating.
Samples were taken at random, regardless of symptoms, to ensure mild and asymptomatic infections were picked up.
The study — which is yet to be peer reviewed — found that two weeks after the second dose, the AstraZeneca vaccine was, on average, 67 per cent effective at preventing COVID-19 infection, and the Pfizer jab was, on average, 80 per cent effective.
The effectiveness of both vaccines reduced over time, though Pfizer's appeared to drop faster. After about 4.5 months, researchers estimated that the effectiveness levels converged.
Another study, published last week in The Lancet, followed 3.4 million Americans vaccinated with Pfizer.
It found the vaccine's ability to protect against infection fell from 88 per cent to 47 per cent over five months, and that the passage of time — not the Delta variant — was the driving factor behind waning effectiveness.
While immunity declined across all age groups at a similar rate, other research suggests age is an important factor when it comes to protection, and how fast it wanes.
In the UK household study, for example, the effectiveness of the AstraZeneca vaccine two weeks after the second dose was 73 per cent for those under 34, compared to 54 per cent for those 35 and over.
But there's still good news
The good news is that despite some waning immunity against infection, COVID-19 vaccines remain highly effective at preventing severe disease and death in most people over time.
A preliminary study recently published by the UK's national health agency looked at the duration of protection provided by the COVID-19 vaccines against mild and severe disease.
It found that although the effectiveness against symptomatic disease started to wane from about 10 weeks, the vaccines continued to provide high levels of protection against hospital admission and death.
"What it tells us is that beyond 20 weeks, you're still getting 95 per cent protection against severe disease and death with Pfizer … and with AZ, it's about 80 per cent," said Tony Cunningham, an infectious diseases physician and clinical virologist at Sydney's Westmead Institute for Medical Research.
The researchers found the decrease in effectiveness appeared to be more common in adults over 65, and people who are immunocompromised.
"In fact, if you take the vulnerable people out of [the AstraZeneca data], you don't see a drop-off in protection against severe disease," Professor Cunningham said.
Another study recently published in the New England Journal of Medicine found protection against hospitalisation remained above 90 per cent for six months among fully vaccinated people in Qatar.
Similarly, the US study that followed 3.4 million Pfizer recipients found protection against hospitalisation held strong at 90 per cent for six months, including among older people.
"If you look at the thing we most care about — am I going to get very sick and end up in hospital? — all of the studies indicate the COVID-19 vaccines are lasting really well," said David Tscharke, a professor of immunology and infectious diseases at the Australian National University.
"Protection against hospitalisation is fantastic."
Understanding changes in antibody levels
Another way researchers assess the effectiveness of vaccines is to look for changes in antibody levels in the blood.
Antibodies are essential to helping us fight off infection, and typically surge following vaccination before naturally tapering off.
"We know [antibodies] are somewhat a proxy for effectiveness, but what we don't know [with COVID-19 vaccines] is exactly how that relationship works," Professor Tscharke said.
Generally, the more neutralising antibodies there are circulating in the blood, the less opportunity the virus has to gain a foothold.
But it's not clear what level of antibodies are required to protect against infection or severe disease.
"The amount to keep you out of hospital is probably much less than the amount needed to stop you getting infected," Professor Tscharke said.
"But how far does that level have to fall before you start losing protection? We don't quite know that point yet."
While several studies have shown that antibody levels decline in the months following COVID-19 vaccination, Professor Tscharke says that's to be expected, and doesn't necessarily mean they'll keep falling.
"With other vaccines, what tends to happen is that decline then starts slowing down … and then levels off," he said.
It may also be that protection against severe disease has little to do with antibodies, and is instead mediated by the body's memory B cells and T cells, Dr Quinn said.
"We may have these other immune mechanisms at play in our body that are well maintained, that might not be waning as much as antibodies over time," she said.
Cell-mediated immunity tends to be long-lasting and more powerful, but is harder to measure.
"We have some good data that suggests memory B cells are being well maintained … but we don't have that information on a really large scale yet," Dr Quinn said.
What about breakthrough infections?
Given that the vaccines aren't 100 per cent effective, we should expect to see some "breakthrough infections" as COVID-19 restrictions begin to lift.
Fortunately, vaccinated people are much less likely to get sick if infected, and very few will require hospitalisation.
That's because if the virus does manage to sneak past the body's first line of defence, it's not long before the immune system — trained by the vaccine — sends in reinforcements to stop it doing serious damage, says Professor Cunningham.
Vaccines, however, don't protect everyone equally. The people most at risk of serious illness from a breakthrough infection are the immunocompromised and the elderly.
"In Australia, almost all the [fully vaccinated] people who have died have been over 70, and almost all of them have had comorbidities," Professor Cunningham said.
"You have to bet quite a number of those people are quite severely immunocompromised, as well as aged."
Last week, the Australian Technical Advisory Group on Immunisation recommended a third dose of a COVID-19 vaccine for people who are "severely" immunocompromised.
Professor Tscharke says it's better to think of these shots as third doses in the initial vaccine schedule, rather than as booster vaccines.
Scaling up vaccine coverage
As for whether the rest of the population will need booster vaccines, most experts agree the more pressing issue is getting as many people inoculated with their first doses as possible.
"[Waning immunity] is a conversation that countries like Israel need to be having, who were early movers and who vaccinated a lot of their population almost a year ago," Professor Tscharke said.
According to the US Centres for Disease Control, unvaccinated people are still the major drivers of transmission.
Research suggests even if fully vaccinated people do get infected, they are less likely to pass the virus on to others.
"Everyone is focusing on the loss of effectiveness of the vaccine," Professor Tscharke said.
"Really, what we should be talking about is just how effective these vaccines are.
"The only protection [from overwhelming the health system] is getting most people vaccinated, so relatively few people end up in hospital.
"By protecting yourself from severe infection, you're actually protecting the health system as well."