Explained: How are Covid-19 vaccines holding up against new variants?

Immunisation Advisory Centre medical adviser Dr Edwin Reynolds described the constantly mutating virus as a “moving target”.

With each new variant – like JN.1, and KP.3, which has been helping fuel a mid-year wave – the virus got better at evading our immune defences and spreading more easily.

But that didn’t mean vaccines were doing nothing to block transmission.

A recent major US study published in the New England Journal of Medicine calculated the Pfizer XBB1.5 vaccine was about 52.2% effective against symptomatic infection after four weeks.

At 10 and 20 weeks, however, that effectiveness dropped to about 32.6% and 20.4% respectively.

IMAC medical adviser Dr Joan Ingram said getting a top-up boosted antibody levels which naturally declined over time – and especially quickly in older people.

“Having a variant-matched vaccine increases antibody protection against those variants by rapidly neutralising the virus early in the infection,” she said.

“As antibody levels wane, the virus may be able to infect our cells but, when we have immune memory, other parts of the immune system step in to kill the virus and generate new antibodies. This takes a little more time, but works to prevent severe disease.”

Does vaccination still protect us from getting seriously sick with Covid-19?

That’s arguably the vaccine’s most important job – and encouragingly, research shows it’s still making a difference.

The same US study showed effectiveness against hospitalisation at four and 10 weeks was 68.8% and 57.1% respectively.

Its effectiveness at preventing death from the virus was higher, but the researchers were less certain about those results given the comparably fewer deaths to assess from.

Another major study, published last week in the journal JAMA Internal Medicine, reported the current vaccine was 62 per cent effective against hospitalisation, compared to being unvaccinated or having received only the original vaccines.

Again, though, the picture was fuzzy.

In an accompanying editorial, researchers said it was now difficult to untangle whether protection was coming from getting the newer vaccine, or simply getting boosted in recent times.

They concluded it was likely both vaccinated and unvaccinated people have some baseline immunity now the virus has been circulating so long.

“There is still some cross-protection from our existing immunity and each exposure boosts our immunity,” Ingram said.

“For healthy people, this is likely to present as a mild respiratory tract infection.”

However, she stressed the need for those people to stay home when sick so they don’t spread it to higher-risk groups.

Who’s most likely to benefit from a top-up?

Ingram singled out several groups who’d benefit from another shot if it’d been more than six months since their last vaccination or infection.

They were those older than 65; Māori or Pasifika people older than 50; severely immuno-compromised people older than 12; or those older than 16 living in aged care or disability care facilities.

Ingram also recommended it to people older than 12 who were eligible for a flu vaccination, such as those who were pregnant, severely obese or underweight, or at extra risk from medical conditions.

“For those with weaker immune responses, any infection takes longer to clear,” she said.

“They also have a weaker response to vaccines so the immunity provided doesn’t last as long: these are the people for whom repeated vaccination is necessary.”

As with the last vaccine, the Government has restricted free eligibility to the current Covid-19 shot to Kiwis aged over 30, with exceptions for high-risk people aged over 12, and those who are pregnant and older than 16.

Health New Zealand – Te Whatu Ora data indicated many eligible Kiwis hadn’t bothered to seek one.

Uptake of the second booster among people in their 30s, 40s and 50s was only about 20% – and just 40% among Kiwis aged 50 to 64.

Fortunately, uptake was much higher among our over-65s, of whom nearly three-quarters have received the second booster.

IMAC medical adviser Professor Peter McIntyre suspected most of those at-risk people dying with the virus hadn’t been recently vaccinated or infected.

In any case, he said the “best bet” for vulnerable people was to meet the virus with as much vaccine-given immunity as possible.

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.