Virus-fighting antibodies have been found in Kiwi Covid-19 patients up to eight months after they were infected - findings that could bode well for the coming vaccine roll-out.
The new research, released ahead of peer-review, could also prove important globally, given the antibodies persisted even when there was no virus circulating in the community.
The study analysed antibodies in a group of 112 New Zealand patients previously infected with the SARS-CoV-2 virus, the bulk of whom suffered mild symptoms.
Antibodies play a critical role in the immune system's fight against pathogens like the coronavirus.
Upon a new virus being recognised, antibodies are specially created to bind to its "spike protein" and stop it entering our cells - all while signalling other parts of the immune system to destroy the foreign invader.
"Because antibodies are very specific to the invading pathogen or virus, they also provide a way to track and study someone's infection history," said Dr Nikki Moreland, an immunologist and biomedical scientist at the University of Auckland.
"In other words, by taking someone's blood sample, and seeing if there are antibodies specific for SARS-CoV-2 in circulation, it's possible to determine if they have previously had Covid-19."
That was useful for diagnostics - especially when there was no longer virus on a swab as the infection was several weeks or months ago.
"By studying the level and function of circulating antibodies, it's also possible to determine if someone has the types of antibodies that might provide protection if they encounter a particular virus or pathogen again."
The new collaborative study conducted by PhD student Alana Whitcombe and research scientist Dr Reuben McGregor in Moreland's team, investigated not just the quantity of antibodies in earlier infected people - but also the quality.
"In particular, do people have antibodies that bind to the virus spike protein, can these antibodies neutralise the virus, and how long do these antibodies last?" McGregor said.
In the laboratory, the researchers measured the level of circulating antibodies that bound to the spike protein, as well as if the antibodies were neutralising.
"As we had samples from people infected months prior we could use these measurements to see how long antibodies were lasting".
"The good news is that we observed that the vast majority of people have neutralising antibodies that bind the spike protein and they could be detected up to eight months after infection."
While overseas studies had shown this too, the key difference was this effect had been shown in a country where Covid-19 had been effectively eliminated.
"People in New Zealand are not being re-exposed to the virus like they are being in countries with high community transmission," Moreland said.
When someone was re-exposed, she explained, their immune system was boosted, which could affect the level of circulating antibodies.
That made similar data from overseas harder to interpret, given it was unclear whether the antibodies were there merely as a result of re-exposure.
"In New Zealand we're fortunate to not have that issue to consider when looking at our data," Moreland said.
"We can be quite confident the antibodies we're measuring are from an initial infection, so seeing these antibodies persist out to eight months is really heartening."
What did that mean for the vaccine roll-out?
Moreland said these studies offered some "positive signals", given that data from vaccine trials showed the agents were inducing similar - and in some cases higher - levels of neutralising antibodies to natural infections.
"So the protection from vaccines is likely to also persist for many months and probably longer," she said.
"But we are still learning in real-time, with each passing month we see that antibodies last one month longer.
"Also, there are several different vaccines and it will be important to track the antibody responses to the various vaccines to gauge if there are differences in the quality and quantity of antibodies they induce, and how long neutralising antibodies to vaccines last."
The study further showed that scientists could accurately measure spike antibodies from a finger-prick blood sample.
"This could drastically improve the feasibility of large-scale studies to track vaccine antibody responses." Whitcombe said.
The paper, uploaded to the pre-print server medRxiv, involved clinicians and scientists from Otago University, the New Zealand Blood Service, Te Punaha Matatini, Callaghan Innovation, the Maurice Wilkins Centre, Southern Community Laboratories, and the Auckland City, Starship and Kidz First Children's hospitals.
"This work would not have been possible without a national network of clinicians, nurses , researchers and scientists and highlights the collaborative nature of New Zealand science during the pandemic," Moreland said.