As Japan's scorching summer ramps up, patients examined for suspected heatstroke have reportedly been diagnosed with COVID-19 instead, highlighting a surge in the infectious disease just as the nation grapples with rising temperatures.

For 10 consecutive weeks, COVID-19 infections have continued to increase, with the number of patients at about 5,000 medical institutions nationwide standing at 55,072 for the week through July 14, up from 11,086 during the week through May 5.

In addition to the spread of new variants such as KP.3 — which is more transmissible and is less susceptible to antibodies from vaccinations or previous infections — the heat also contributes to weakened immune systems, exacerbating the situation.

Hiroyuki Moriuchi, head of the department of pediatrics at the Nagasaki University Hospital, said that people should avoid trying to self-diagnose, as getting a fever and feeling lethargic are symptoms of both heatstroke and COVID-19.

There are many illnesses with similar symptoms, but during this season, two common causes are heatstroke and COVID-19, he said.

“Without proper testing, especially viral testing, it can be difficult to diagnose,” Moriuchi said, adding that sometimes, following a thorough examination of the mouth, it transpires that patients with COVID-like symptoms actually have hand-foot-and-mouth disease.

Although home testing kits are now readily available, he said that proper sample collection is crucial, as the same test kit might show a positive result when administered in a hospital, but could yield a negative result when used at home.

When you're not feeling well and your overall immune function is weaker, there can be a difference in how your body responds to the same virus — whether it goes unnoticed or symptoms develop — he added.

“The main reason for the increase in infections during summer is not just the season itself,” he said. “But rather the fact that summer creates an environment where person-to-person transmission is more likely.”

The urge to stay indoors, close the windows and use air conditioning — important steps for people vulnerable to heatstroke and at certain temperatures — creates a confined space with poor ventilation, making it easier for infections to spread.

“For now, the priority is to protect high-risk individuals, such as the elderly and those with underlying conditions,” he said. “When you’re around people at higher risk, it’s important to ensure good ventilation and wear masks when talking close to them.”

Japan continues to see dangerous temperatures of 37 degrees Celsius and above in numerous places — there were 6,194 emergency transports nationwide due to heatstroke in the week through July 14 — and the risks for heatstroke and COVID-19 often overlap, particularly for the elderly, who are most at risk.

High-risk individuals should be able to access medical care immediately, Moriuchi said, but to ensure that, it's important that not-too-serious COVID-19 cases do not overwhelm hospitals and that young and healthy individuals manage their situations calmly and avoid unnecessary panic.

“If you notice any unusual symptoms, whether you’re young or elderly, it’s important to visit a hospital or call an ambulance if needed,” he said.

Moriuchi emphasized the need to balance the prevention of infections with continued social activities, as mental well-being is crucial. And for the elderly, being separated from their relatives or grandchildren for an extended period of time can have significant negative effects.

Individuals should make their own judgment on whether to limit outings, wear a mask or attend social gatherings, Moriuchi said.

“Making these decisions and making informed choices is essential for all of us to coexist with COVID-19,” he said.