Victorians can now receive their second AstraZeneca vaccine dose from six weeks rather than after three months, given the outbreak in the state. New South Wales made a similar announcement in August, with people able to receive their second dose of AstraZeneca anywhere from four to eight weeks after the first.
It has left some asking whether reducing the interval between doses will lower their protection against the Covid-19 virus. So what are the implications of getting fully vaccinated sooner?
The official advice
Australia’s Technical Advisory Group on Immunisation (Atagi) supports reducing the dose interval when outbreaks are occurring.
Atagi, which is comprised of leading infectious diseases experts and epidemiologists, says an interval of between four and eight weeks between the first and second doses is preferred in an outbreak situation. In non-outbreak settings, the preferred interval remains at 12 weeks, because early clinical trials (before Delta was on the scene) found higher vaccine efficacy at protecting against symptoms.
The data which informed the 12-week recommendation came from a study in The Lancet, which found leaving less than six weeks between the first and second dose gave 55.1% efficacy at protecting against any symptoms, including mild ones. A six-to-eight week gap increased efficacy to 59.9%, and at 12 weeks or longer efficacy rose to 81.3%. But this study was conducted before the Delta variant existed.
Associate Professor Nathan Bartlett, the head of the viral immunology and respiratory disease group at the University of Newcastle, has said people should be “really cautious” about extrapolating information about vaccine effectiveness from studies conducted before Delta arrived.
What data do we have on Delta and AstraZeneca?
Real-world data is showing that the AstraZeneca vaccine, whether given at six or 12 weeks apart, or somewhere in between, is “offering very good, very high levels of protection against hospitalisation and severe disease from Delta”, Bartlett said. “It’s certainly clear that it doesn’t really matter when you had those two doses in terms of protecting against severe disease, just as long as you’ve had two.”
According to Atagi, data so far on AstraZeneca and the Delta variant coming from overseas is showing even a single dose of AZ reduces the risk of hospitalisation by 71%. Two doses cuts the risk of hospitalisation by 92%.
Most vaccines work in this way; they’re more effective at preventing severe disease than mild disease. This is why focussing too heavily on how effective the vaccines are at preventing any disease at all, even mild symptoms, when given at six weeks or 12 week intervals is not the key data people should be focussed on.
Protection against severe disease and death is what is crucial, and data is showing the AstraZeneca vaccine offers strong protection against that, including when it comes to the Delta variant.
“It is just that we’ve just been so obsessed with numbers right from the beginning, whether it’s rates of clotting or effectiveness, and we are just losing sight of the big picture,” Bartlett said. “The key reason we’re doing this is to keep people out of a hospital, and so the health system doesn’t implode.”
A professor of immunology at Victoria University, Vasso Apostolopoulos, said it would be worrying if people believed reducing the interval between doses significantly lowered their protection against becoming seriously unwell.
“The dosing interval really doesn’t matter,” she said.
“People in Victoria and New South Wales shouldn’t worry that they’re not going to get as good an immune response. There’s an urgent need for them to get vaccinated right now, as soon as possible, because we need to control this.”
What about waning protection over time?
Bartlett said based on studies that have been conducted in the UK on vaccine effectiveness since the emergence of Delta, “it’s clear that after six months from vaccination there is a waning of vaccine-induced immunity, there’s no question about that”. This is true of all the Covid-19 vaccines, not just AstraZeneca.
“But that’s not a huge issue in the immediate term, because you’re starting off at such a high level of protection against severe disease and death, upwards of 90%,” he said. “So you can afford to lose 15 or 2o% of that and still be very protected.” Effectiveness seems to wane at similar rates across the vaccine types.
However, if that trajectory continues further downwards over time, then the level of breakthrough infections and virus circulating in the community is going to increase, Bartlett said.
“That’s exactly what we’re seeing in the UK, and it is exactly what we’re seeing in Israel, who began vaccinating much sooner,” Bartlett said.
“Case numbers in the UK are going up to the highest they have been since March, and deaths are slowly seriously creeping back up. I think that’s just indicative of that gradual waning of protection in the overall population. It has allowed a little bit more virus to circulate with a little bit more frequency so those who are most vulnerable and maybe don’t have as good protection from the vaccine, such as the immunocompromised, are more exposed.”
What does this mean for a booster dose of the vaccine?
This waning immunity, combined with the higher infectiousness of Delta and relaxed restrictions, is why countries such as Israel, where the majority of adults had been fully vaccinated with Pfizer by April, are now offering booster doses to its population. The UK and the US are doing the same for those who are immunocompromised and who have a less strong response to a two-dose schedule.
Bartlett said Australia still has some time before it needed to start administering booster shots, but it was something that health authorities would need to consider. Other countries had vaccinated faster and sooner, so they were seeing reduced protection sooner.
“We just need to get everyone in Australia fully vaccinated with two doses in the first place, and then boosters are a conversation you have once you’ve got everybody fully vaccinated,” Bartlett said.
“You might have that conversation about a third dose now for those who are immunocompromised – so organ transplant patients and cancer patients.
“But for the rest of us, I think we need to park that for now. Next year though, we can discuss the strategy for maintaining our level of protection, which will certainly involve boosters.”