Wed. Aug 10th, 2022

Worldwide, COVID-19 vaccines work remarkably well to reduce the number of serious illnesses and deaths.

However, an increasing number of studies suggest that their ability to protect humans from mild illness and asymptomatic infections diminishes over time. But how much?

It’s hard to “put a hard number on it,” says vaccine researcher Kylie Quinn.

“We have moved from these nice, tidy randomized control trials … [to] have to measure it on the run, “said Dr. Quinn of RMIT University.

For researchers, it has been difficult to tease out how much of what they see is the result of declining immunity, and how much comes down to the Delta variant, easing restrictions and the vulnerability of people who are vaccinated early.

“I think it’s clear that Delta is more of a challenge for these vaccines,” Quinn said.

“But now we’re starting to get cleaner datasets that show that there’s an extra challenge with declining immunity.”

How much does this mean? And what does it mean when Australia starts to open up?

Let’s see.

Evidence shows protection against getting COVID diminished over time

To investigate how well COVID-19 vaccines work to protect against infection, British researchers recently estimated more than 350,000 COVID-19 test results taken as part of a large household study between May and August this year, in which the Delta strain dominated.

Samples were taken randomly, regardless of symptoms, to ensure that mild and asymptomatic infections were collected.

The study – which has not yet been peer-reviewed – found that two weeks after the second dose, the AstraZeneca vaccine was on average 67 percent effective in preventing COVID-19 infection, and the Pfizer jab was on average 80 per cent. cents effective.

The efficacy of both vaccines was reduced over time, although Pfizers appeared to decline more rapidly. After approximately 4.5 months, researchers estimated that efficiency levels converged.

Similar declines in efficacy against infection have been reported in Qatar, Israel and the United States, which were among the first countries to roll out COVID-19 vaccines on a large scale.

One vial with Moderna, one with AstraZeneca and two with Pfizer are arranged on a plain gray background.
Vaccine efficacy is generally slightly higher in younger people compared to older age groups.(Shutterstock: Marc Bruxelle)

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 dropped from 88 percent to 47 percent over five months, and that the passage of time – not the Delta variant – was the driving factor behind declining efficacy.

While immunity declined in all age groups at a similar rate, other research suggests that age is an important factor when it comes to protection and how quickly it declines.

In the UK household study, for example, the effectiveness of the AstraZeneca vaccine two weeks after the second dose was 73 per cent for people under 34 years of age compared to 54 per cent for those aged 35 and over.

But there is still good news

The good news is that despite a weakened immunity to infection, COVID-19 vaccines remain extremely effective in preventing serious illness and death in most people over time.

A preliminary study recently published by the UK National Health Agency looked at the duration of protection from COVID-19 vaccines against mild to severe disease.

It found that although the effectiveness against symptomatic disease began to decline from about 10 weeks, the vaccines continued to provide a high level of protection against hospitalization and death.

“What it tells us is that after 20 weeks you still get 95 percent protection against serious illness and death with Pfizer … and with AZ it’s about 80 percent,” said Tony Cunningham, an infectious disease doctor and clinical virologist at Sydney Westmead Institute for Medical Research.

A man in a white coat standing in front of a sign.
Professor Cunningham is an internationally renowned physician, infectious disease, clinical virologist and scientist.(ABC News: Chris Taylor)

The researchers found that the decline in efficacy appeared to be more common in adults over 65 years of age and people who are immunocompromised.

“In fact, if you take out the vulnerable people [the AstraZeneca data], you do not see a decrease in the protection against serious illness, “said Professor Cunningham.

Another study, recently published in the New England Journal of Medicine, found that hospitalization protection remained above 90 percent for six months among fully vaccinated people in Qatar.

Similarly, the U.S. study that followed 3.4 million Pfizer recipients found hospitalization protection remained strong at 90 percent for six months, including among the elderly.

When it comes to the Moderna vaccine, research suggests the plug is even more effective than Pfizer.

“If you look at what we like most – will I get very sick and end up in the hospital? – all studies indicate that the COVID-19 vaccines hold up really well,” said David Tscharke, professor of immunology and infectious diseases at the Australian National University.

“Protection against hospitalization is great.”

Understand changes in antibody levels

Another way researchers are evaluating the effectiveness of vaccines is to look for changes in antibody levels in the blood.

Antibodies are essential to help us fight infection and typically rise after vaccination before they naturally subside.

“We know [antibodies] is a bit of a proxy for efficiency, but what we do not know [with COVID-19 vaccines] that’s exactly how that relationship works, “Professor Tscharke said.

In general, the more neutralizing antibodies circulating in the blood, the less chance the virus has of gaining a foothold.

However, it is not clear what level of antibodies are required to protect against infection or serious illness.

“The amount to keep you out of the hospital is probably much less than the amount needed to stop you from getting infected,” Professor Tscharke said.

“But how far should that level drop before you start losing protection? We don’t quite know that point yet.”

Although several studies have shown that antibody levels fall in the months following COVID-19 vaccination, Professor Tscharke says that is to be expected and does not necessarily mean that they will continue to fall.

“With other vaccines, it tends to happen that the fall then begins to slow down … and then goes out,” he said.

It may also be that protection against serious illness has little to do with antibodies, and instead is mediated by the body’s memory B cells and T cells, said Dr. Quinn.

“We may have these other immune mechanisms at play in our well-maintained body that may not diminish as much as antibodies over time,” she said.

Cell-mediated immunity tends to be long-lasting and more powerful, but is more difficult to measure.

“We have some good data suggesting that B memory cells are well maintained … but we do not have that information on a very large scale yet,” Quinn said.

What about breakthrough infections?

Given that the vaccines are not 100 percent effective, we should expect to see some “breakthrough infections” when COVID-19 restrictions begin to lift.

Fortunately, vaccinated people are far less likely to get sick if they become infected, and very few will require hospitalization.

That’s because if the virus manages to sneak past the body’s first line of defense, it’s not long before the immune system – trained by the vaccine – sends in reinforcements to stop it from doing serious damage, Professor Cunningham said.

However, vaccines do not protect everyone equally. The people most at risk for serious illness due to a breakthrough infection are immunocompromised and elderly.

“In Australia, almost everyone is [fully vaccinated] people who have died have been over 70 and almost all have had comorbidities, ”said Professor Cunningham.

“You have to bet that some of these people are quite severely immunocompromised as well as elderly.”

Last week, the Australian Technical Advisory Group on Immunization recommended a third dose of a COVID-19 vaccine for people who are “severely” immunocompromised.

Professor Tscharke says it is better to think of these shots as third doses in the initial vaccination plan rather than as booster vaccines.

Upscaling of vaccine coverage

A food delivery woman rides a bicycle while wearing a mask in Sydney.
Australia’s vaccination program has gained momentum since local outbreaks in several states threw millions into lockdown.(Getty Images: Jenny Evans)

As for whether the rest of the population will need booster vaccines, most experts agree that the more urgent problem is to get as many people inoculated with their first doses as possible.

“[Waning immunity] is a conversation that countries like Israel should lead, which were early migrants and which vaccinated much of their population almost a year ago, “said Professor Tscharke.

According to the US Centers for Disease Control, unvaccinated people are still the biggest carriers of transmission.

Research suggests that even if fully vaccinated people become infected, they are less likely to transmit the virus to others.

“Everyone is focusing on losing the 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] get most vaccinated so relatively few people end up in the hospital.

“By protecting yourself from serious infection, you are actually also protecting the health care system.”

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Casey Briggs looks at national vaccination rates

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