Experts are suggesting people perhaps shouldn't be considered fully vaccinated against COVID-19 until they've had three doses.
Early evidence suggests two doses aren't enough to protect against the new Omicron variant, but three might be; and a new study has found a booster shot can divide the risk of infection with Delta by 10, and the risk of serious illness by 18, at least in the short term.
Researchers in Israel looked at data covering nearly 4.7 million people aged 16-plus in the Middle Eastern nation, which had one of the earliest vaccine rollouts but suffered a huge wave of Delta variant infections in mid-2021.
It quickly became clear protection offered by the Pfizer-BioNTech vaccine was weakening over time, at least against infection. The outbreak was rapidly brought under control when Israel began a booster shot campaign in July, the first country in the world to do so.
Others - including New Zealand - have since followed.
Analysis of Ministry of Health data published in the New England Journal of Medicine on Thursday found people who'd got a booster shot were 10 times less likely to be infected than those with just the standard two-shot course, and 18 times less likely to get a severe infection.
Among the at-risk over-50s, booster shots were associated with a 90 percent reduction in mortality, and for over-60s, 93 percent.
This, combined with the Omicron variant's apparent ability to evade vaccine protection without a booster, has scientists wondering if the definition of fully vaccinated should now be three doses - especially with much of the world, including New Zealand, trying to open up.
"The COVID-19 Protection Framework is designed around an assumption that a person with two doses of the Pfizer vaccine is fully vaccinated," said Amanda Kvalsvig, an epidemiologist at the University of Otago, Wellington.
"We're now seeing evidence that we shouldn’t be considered fully vaccinated until we've had three doses."
BioNTech CEO Uğur Şahin said on Wednesday countries should consider bringing forward their planned booster shots from six to three months, as the UK has.
"We believe this is the right way to go particularly if the Omicron is now spreading further, to enable a better level of protection in the winter season."
Some US authorities and organisations have updated their definitions to count three doses as fully vaccinated - including the NBA and New Mexico. The top US infectious disease expert Anthony Fauci said last week while the official definition remains two doses, that could change thanks to Omicron.
Pfizer has said it might have an Omicron-specific formula ready by March.
While protection against infection drops significantly six months after the second dose, so far it seems protection against severe illness holds up.
Dianne Sika-Paotonu, immunologist at the University of Otago, Wellington, said the latest data - particularly that on Omicron - suggests a third dose is "needed".
"The antibody levels generated against Omicron after three Pfizer-BioNTech COVID-19 vaccine doses was comparable to the protection seen towards the wild-type and other variants of the SARS-CoV-2 virus after two Pfizer-BioNTech COVID-19 vaccine doses.
"The third Pfizer-BioNTech COVID-19 vaccine dose also showed stronger T cell immune responses were still generated, which is important as T cells are considered to indicate protection against severe COVID-19 disease."
Dr Kvalsvig said with Omicron likely to arrive in New Zealand ahead of winter, relying on double-dose vaccine coverage for access to events, bars and hairdressers - all close-contact situations linked to transmission - was a "high-risk strategy".
"To keep New Zealanders safe this summer and beyond we need layered protection that recognises the critical role of airborne transmission in this pandemic... There needs to be strong public health messaging and support from Government for meeting up outdoors, keeping windows and doors open, and using ventilation and filtration systems in schools, workplaces, and other crowded indoor settings."
Without a comprehensive plan, New Zealand faces "a difficult winter of dealing with colder weather, new COVID-19 variants, declining vaccine protection, and a potentially very high burden of flu and numerous other infections returning as the borders open", Dr Kvalsvig said.
