The world may have its first case of a person being infected with the coronavirus twice - and scientists say it's not yet clear what that means for New Zealand, or our hopes for an early vaccine.

Hong Kong University researchers this week reported the case of a 33-year-old man who caught the virus again less than five months after being infected the first time.

The man was reported to have mild symptoms after the first episode, in March, after which no detectable antibodies were detected in his blood.

In the second infection - which was picked up during screening at Hong Kong airport after he spent time in Spain, then returned via London - he had no symptoms, yet did produce antibodies.

Genomic sequencing showed the two genetic lineages were separate.

The study reporting the case hasn't yet been published in a scientific journal, meaning its findings need to be treated with caution.

The University of Otago's Dr Jemma Geoghegan said the sole case of re-infection needed to be treated with caution. Photo / Otago University

But one of its authors, prominent researcher Professor Yuen Kwok-yung, said it raised potential implications for hopes of herd immunity against Covid-19.

"First, it is unlikely that herd immunity can eliminate [the pandemic], although it is possible that subsequent infections may be milder than the first infection, as for this patient," he said.

"Covid-19 will likely continue to circulate in the human population as in the case of other human coronaviruses."

Asked what the case meant for New Zealand, Director-General of Health Dr Ashley Bloomfield told the Herald it was "too early to read anything into that finding", adding the World Health Organisation had shared the same view.

Scientists here and overseas have also echoed that point, noting the one instance of re-infection stood among more than three million cases so far reported worldwide.

"It doesn't mean this is going to happen to everyone," Otago University and ESR virologist Dr Jemma Geoghegan said.

"So we really don't know what this means for New Zealand. Because we don't know if this is related to a lack of antibodies, or whether the person had any underlying health conditions, we can't extrapolate this to a broader population.

"I think it's important to remember that we have only known about this virus for eight months now, and are still learning a lot about it."

Sequencing work led by scientists such as Geoghegan has already ravelled much about the virus, its many lineages, and how far these can spread around the globe.

While New Zealand has recorded relatively few cases of Covid-19 when compared with other western nations, sequencing has shown what infections the country has had represented virtually all of the virus' genetic global diversity.

Associate Professor James Ussher, of Otago University's Department of Microbiology and Immunology, said reinfection with endemic human coronaviruses - which happen to cause a large proportion of colds - was well described.

Therefore, it wasn't surprising the same could happen with this new coronavirus, SARS-CoV-2.

"It remains unknown how common reinfection will be, what will be the median duration between infection and reinfection, and whether the clinical course of patients who are reinfected with SARS-CoV-2 differs from primary infection."

For instance, he said, did that mean asymptomatic or mild disease would prove more common? Or could severe disease still occur a second time?

"It complicates the idea of achieving herd immunity through natural infection."

Immunisation Advisory Centre director Dr Nikki Turner also saw some important factors relating to the Hong Kong case.

"For example, people who are on medications that may affect their immune response, or have certain medical conditions that affect their immune response, may have a shorter duration of protection than others," she said.

"When someone does get sick again from the same virus, it is usually more mild as there is some existing immunity, but the immunity is not adequate to fully protect. I understand that is what was seen in this case."

Immunisation Advisory Centre director Dr Nikki Turner sees several important factors relating to the Hong Kong case. Photo / Mark Mitchell

What did the new case mean for the race toward a Covid-19 vaccine?

Dr David Strain, chair of the British Medical Association's Medical Academic Staff Committee, felt it did raise some troubling questions.

"Vaccinations work by simulating infection to the body, thereby allowing the body to develop antibodies," he said.

"If antibodies don't provide lasting protection, we will need to revert to a strategy of viral near-elimination in order to return to a more normal life."

University of Auckland vaccinologist Associate Professor Helen Petousis-Harris, however, was hesitant to make assumptions.

She said it might not actually prove to have much bearing on those vaccines now under development, for the simple reason they didn't need to mimic natural infection.

"In the case of this coronavirus, natural infection actually interferes with the immune response, something that vaccines will not have to contend with," she said.

"I am inclined to keep an open mind and not jump to conclusions."

While the report is held to be the world's first confirmed re-infection, several other papers have also pointed to suspected cases - although none have been backed up through additional sequencing.