AstraZeneca vaccine: how protected will it be if getting 2nd dose sooner?

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. 

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. 

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.

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.