Symptoms of influenza A(H1N2)v, clade 1b.1.1, were mild in the individual, who has fully recovered, health authorities said in a news release. Hundreds of documented cases of swine flu occur globally on an annual basis, each transmitted from a pig to a human, and typically no further.

But authorities are unsure of how the U.K. patient became infected, leaving open the possibility that the virus is spreading in the community. And mild symptoms in one patient aren’t a guarantee of mild symptoms in others if they become infected, experts tell Fortune.

The HSA is working to assess the risk to humans, searching for other possible cases, and increasing surveillance in health facilities in parts of North Yorkshire, as part of an ongoing investigation.

“We are working rapidly to trace close contacts and reduce any potential spread,” Meer Chand, incident director at the HSA, said in a statement.

In the meantime, it recommends that those with general symptoms of respiratory viruses like the following avoid contact with others, particularly those who are elderly or have medical conditions:

• continuous cough
• high temperature, fever, or chills
• changes in senses of taste and/or smell
• shortness of breath
• unexplained fatigue
• muscle aches and pains not due to exercise
• lack of appetite
• headache longer lasting/more severe than usual
• sore throat
• stuffy or runny nose
• diarrhea

Apparent first human swine flu case of its exact kind

The infection marks the first time the exact type of H1N2 virus has been detected in humans, though an incredibly similar strain has been detected in area pigs. There have been a total of 50 human cases of H1N2 reported globally since 2005. But none are genetically related to the clade, or subgroup, just detected in the U.K.: 1b.1.1.

A single case of the new flu clade isn’t concerning to Rajiv Chowdhury, professor of global health at the Robert Stempel College of Public Health and Social Work at Florida International University in Miami.

What concerns him, however, is the possibility of cases going undetected outside of the the U.K., which has far better genomic surveillance capabilities than most other countries in the world.

While it’s reassuring that the individual experienced only a mild course of illness, it doesn’t rule out the possibility of a more severe course in other humans, if they’re infected. Factors that can influence the severity of a disease course include age, other medical conditions, immunity status, and the amount of virus they were exposed to, Chowdhury told Fortune.

And features of a virus can change with time, and as the number of reports potentially increases. Mortality rates during the H1N1 pandemic of 2009 initially appeared higher in Mexico, but dropped with time as the number of cases grew, Dr. Bruce Y. Lee, professor of health policy and management at the City University of New York School of Public Health, told Fortune.

Dr. Michael Osterholm—director of the University of Minnesota’s Center for Infectious Disease Research and Policy (CIDRAP)—agreed with Chowdhury, telling Fortune that sporadic cases of swine flu are not unexpected. He pointed to the lack of report of human-to-human transmission so far as an encouraging sign.

Hundreds of cases of swine flu occur in the U.S. each year, usually linked to agricultural fairs, Dr. Amesh Adalja, an infectious disease specialist and senior scholar at the Johns Hopkins Center for Health Security, told Fortune. An 18-year-old was infected with A(H1N2)v in Michigan in this summer after visiting an agricultural fair, according to the U.S. Centers for Disease Control and Prevention. He recovered outside of the hospital and apparently did not infect any others.

“Most of these viruses do not have the capacity to spread between humans,” Adalja said. “So it will be important to understand the transmission chain that led” to the human’s infection, including if that human had any exposure to pigs.

If not, the case could signal community spread.

‘Isolated case or the tip of an iceberg?’

One difficult aspect of tracing flu spread is that symptoms can be unspecific, and asymptomatic carriers can spread the virus without ever knowing they’re sick, said Lee, who was embedded in the U.S. Department of Health and Human Services during the country’s H1N1 pandemic response.

“Whenever we see cases like this, the big question is always, is this an isolated case or the tip of an iceberg?” he said.

Humans are occasionally infected with the swine flu by pigs, usually after exposure to pigs or contaminated environments. Even more rarely, the virus transmits from an infected human to other humans, as was the case during the H1N1 pandemic of 2009. The H1N1 flu virus was a combination of bird, swine, and human flus, and now circulates as a seasonal flu.

The detection of any new variant of flu “warrants further investigation to assess the pandemic potential and to look for similar or related cases,” Nita Madhav, senior director of epidemiology and modeling at Boston-based biotech firm Ginkgo Bioworks, told Fortune.

Monday’s report of a new H1N2 clade makes the case for heightened genomic surveillance worldwide, even after the COVID pandemic, she said.

Chance of another pandemic soon ‘similar to flipping a coin’

Madhav is coauthor of a paper released this month by the Center for Global Development, which found a high probability of another pandemic on the scale of COVID in the next quarter-century—in particular, a respiratory pandemic like a flu or coronavirus. Regardless, her team expects such viruses to cause at least 2.5 million deaths annually.

“Our simulations suggest that an event having the mortality level of COVID-19 should not be considered a ‘once in a century’ risk,” Madhav and colleagues wrote. “To the contrary, over the next 25 years, we estimate that a pandemic with magnitude similar to or worse than COVID-19 has a roughly 50% probability of occurrence, similar to flipping a coin.”

Without adequate monitoring, humans may have no clue if a swine flu, bird flu, or other pathogen is regularly spilling over from animals to humans, and could be blindsided by the next pandemic, Madhav said.

The data released so far on the new clade of swine flu is reassuring, Dr. Stuart Ray, vice chair of medicine for data integrity and analytics at Johns Hopkins’ Department of Medicine, told Fortune.

“If this particular H1N2 variant spreads rapidly and causes more severe or widespread infections in people with the seasonal flu vaccine, that might drive additional public health measures,” he said. “For now, it’s wise to recognize the value of high-quality surveillance.”