Coronavirus: Are COVID-19 vaccine booster shots needed? What the science says

The Opposition says it would be "unforgivable" if the Government doesn't urgently order booster shots of the Pfizer COVID-19 vaccine, but scientists aren't so convinced. 

"We all accept elimination for now, but unless you do things like get onto the booster jabs, we'll just keep having lockdowns into next year," National MP Simon Bridges told The AM Show on Friday. 

"Booster jabs - these guys are not buying them at the moment when other countries are," said Bridges. "We will look back on that next year as unforgivable. We accept we're in lockdown, that's the way it is given the slowness, but you know, let's get on with the job now - including getting those booster jabs."

Some countries whose vaccine rollouts started earlier than ours have either started giving third doses of COVID-19 vaccines, or are planning to soon. Israel's already rolled out booster shots to over-60s, and has started offering them to people 30 and over whose second dose was at least five months earlier, Reuters reported earlier this week. The US plans to start boosters in September. 

As a new type of vaccine against a new disease, the long-term effectiveness of the Pfizer jab - or any other of the COVID-19 vaccines on offer - is unknown. The Pfizer vaccine uses a new technology called mRNA - rather than injecting the body with weakened or deactivated viral material, the body's cells are instead taught how to make a part of the virus called the spike protein, and learns to recognise it and fight it this way. 

Studies have found the level of antibodies against COVID-19 from the Pfizer jab wane over time, and booster shots can bring them back up.

"Boosters are given to stimulate immune memory," Helen Petousis-Harris, vaccine safety and effectiveness researcher at the University of Auckland and former chair of the World Health Organization Global Advisory Committee on Vaccine Safety, told Newshub earlier this week

It's not known how much more time booster shots will give, with some experts saying they may be needed annually to keep antibody levels up - and others suggesting they may not be needed at all.

"I don't think we have enough data just yet to be sure whether we'll be needing boosters, or when," Fran Priddy, clinical director for Vaccine Alliance Aotearoa New Zealand, told Newshub. 

"There is quite a bit of data coming up in the next several weeks to months that may give us a better sense of whether we truly need them."

The vaccine was developed to prevent serious illness, hospitalisations and death - and the evidence so far is, even with the Delta variant, they still do that. 

"You can see antibody levels decline but you can still be protected form a disease - you might get an infection but not get sick, for example - antibodies are not our only defence," said Dr Priddy, adding that it's quite normal for antibody levels to decline over time following a vaccine or infection. 

The body's immune system also has memory cells. These remember what the virus looks like, and produce new antibodies if they detect an intrusion by the virus. A study published this week by researchers at the University of Pennsylvania found while the number of antibodies - which can quickly neutralise the virus - declines considerably in the six months after the second dose, the number of memory B cells goes up and they become better at recognising the virus, even in variant forms. 

"Your immune system has a backup," said study leader John Wherry.

These memory cells aren't as quick to react as antibodies, so it can take them a few days to "kick into action and prevent severe disease", Dr Wherry said - perhaps explaining why vaccinated people are far less likely to fall sick from Delta, but can still pass it on.

A strong antibody response immediately stops the virus from getting a foothold, preventing both illness and infectivity; while a delayed memory cell response will stop people getting sick, since it takes days for symptoms to develop after an infection, they can still carry the virus for a few days and pass it onto others. 

"We're seeing the levels of antibody that vaccinated patients have declined over time, which can sometimes signal a loss in protection," said Jesse L Goodman, a professor of medicine at Georgetown University in the US. 

"What we’re not seeing yet, and remains in question, is whether this has an effect on the outcomes we’re really trying to prevent, which is hospitalisation, severe disease, death. And so far vaccine protection against those more severe manifestations is holding up well."

Another expert - Ali Ellebedy, associate professor of pathology and immunology at Washington University School of Medicine - said the evidence in favour of booster shots so far was collected just a month after the third dose was given, "which would be the time antibody production is highest". 

"Two main questions remain: First, how sustained is the increase in antibody levels? Ideally it would last six months or more. Second, would that third shot impact the spread of highly infectious variants, such as Delta?"

The Government opted to use only the Pfizer jab in the initial rollout after observing its effectiveness overseas, while some others ran into safety and efficacy concerns. This meant starting later than most other developed countries.

But Dr Priddy said rather than being behind the curve, New Zealand is in a good position - we don't have widespread transmission of the virus compared to Israel and the US, so can wait and see how booster shots go overseas before making a decision.

"Yes, we're having a Delta outbreak, but we just are not seeing the levels of case numbers other countries are seeing. So we should still be thinking about boosters, we'll need them following the data, but the primary issue in New Zealand is getting everyone their first vaccinations." 

The bulk of New Zealanders will be getting their doses over the next few months. By the time we'd even consider boosters, the data on whether they're effective should be in, Dr Priddy said.

The Government has ordered 10 million doses of the Pfizer vaccine. Since not everyone will be eligible or want the vaccine, it has said there will be some left over to start a booster programme. Orders for other vaccines from Novavax, AstraZeneca and Jannssen are also in place, in case they're needed. 

"Are people who already received the vaccine six to eight months ago... are the rates of hospitalisation going up?" said Dr Priddy. "That might be a sign for us that we might need to institute boosters." 

The World Health Organization has recommended against booster shots, saying not only is the evidence for them lacking, there are billions of people in poorer nations who haven't even had their first doses yet. Widespread outbreaks in those places would increase the chance of an even fitter variant than Delta emerging.