There are known unknowns. How many people would need them or how regularly? How long does vaccine immunity generally last? Can they reduce the threat of variants?
So far boosters are mainly being used overseas to help elderly or medically vulnerable people. The United States Centers for Disease Control and Prevention say a third dose can benefit people with weak immunity.
An Israeli study showed that an extra jab of the Pfizer/BioNTech vaccine allowed immunosuppressed people to develop 43 per cent more antibodies than previously.
The World Health Organisation is concerned that vaccines need to be far more widely dispersed around the world first.
Wealthy countries with extensive rollouts are concerned about Delta case spikes thanks to large pockets of unvaccinated people and have previously booked jab supplies for the next two years.
Vaccine-makers argue that vaccinated people will need boosters at some stage and say mRNA vaccines can be updated to target a particular strain.
Former CDC director Tom Frieden tweeted: "Waning immunity MAY occur. And it MAY be the case that we will need tweaked vaccines because of variants. We don't know yet. The data on waning immunity isn't there, although some preliminary results from Israel and elsewhere hint that either Delta or waning immunity may account for some breakthrough infections. It will be important to continue monitoring vaccine safety and effectiveness going forward. Bottom line: Our vaccines are doing their job."
Data and studies overseas continue to show strong vaccine effectiveness in preventing deaths and severe illness requiring hospitalisation. Delta flourishes among the unvaccinated and partially vaccinated.
But vaccines are less able to prevent basic Delta infection and some transmission. The vaccines need help in the form of high vaccine take-up, mask-wearing in high-risk situations and social distancing to do that.
Australia's Delta outbreak shows how the coronavirus has advanced and is able to exploit opportunities. A still worse variant could emerge.
The situation in New South Wales has worsened over two months, despite restrictions in greater Sydney. On Saturday the entire state was put under lockdown. On Sunday 415 new cases and four deaths were recorded.
Lockdown and contact tracing as tools appear to have been blunted there.
That's in part because Delta is a greater danger to younger people. More than a quarter of cases in NSW have been in the under-20s age group. Vaccination rollouts have tended to focus first on the elderly and most vulnerable with the young last in line.
Australia is receiving an extra million doses of the Pfizer/BioNTech vaccine from Poland and is focusing on vaccinating more people aged under-40 in Sydney. NSW Premier Gladys Berejiklian said: "They are the ones that are mobile because of their work. They are the ones who cross three generations, and they are ones who were part of the 70 per cent of cases we are currently experiencing in those most problematic areas."
New Zealand hopes to avoid any outbreaks until most people are vaccinated and then start a phased, careful reopening. By then it will have been a year since the first Kiwis had their shots.
Booster doses for Covid, like annual flu jabs, may be a drug company's profit-stream dream but arguably they make sense for economic and healthcare stability.
Vaccines regularly tweaked to target major variants could be a positive help in avoiding restrictions in the future.
The slow vaccine supplies caused political angst. With this tricky virus it seems better to be over-prepared than to underestimate.
