Everything you need to know about BA.5, the dominant strain spreading across Japan

In the week through Sunday, Japan logged more new COVID-19 cases than anywhere else in the world.

Amid a seventh wave of infections, daily case numbers have been reaching unprecedented highs, with the nation logging 969,000 cases over the seven-day span to Sunday. By comparison, after the first COVID-19 case was detected in Japan, it took 19 months for the country to log its 1 millionth case.

Driving the surge is a coronavirus subvariant called BA.5, which has recently overtaken other omicron subvariants to become the dominant strain worldwide.

So how unique is this subvariant and what can you do to stay safe? Here’s what you need to know about BA.5:

What is BA.5?

Omicron, which emerged late last year, has produced a number of subtypes. They include the original omicron subvariant BA.1, as well as BA.2, BA.3, BA.4 and BA.5.

Of over 200,000 coronavirus samples submitted to an international virus data sharing platform between mid-June and mid-July, BA.5 represented 53.59% of all omicron sequences, followed by 10.57% for BA.4 and 4.51% for BA.2.

BA.4 and BA.5 are often discussed together because they feature identical mutations in their spike protein, which allows them to more easily latch onto and enter cells. Through these mutations the two subvariants are more infectious than previous strains of the coronavirus and can also escape immunity gained through vaccinations and past infections, experts say.

According to the National Institute of Infectious Diseases, BA.5’s effective reproduction number — or the average number of secondary infections caused by an infected individual — based on Tokyo data was about 1.27 times that of BA.2, a previously dominant subvariant. Omicron in general spreads faster than delta; its generation time, or the time between primary and secondary infections, is estimated at around two days, compared with delta’s five days.

BA.5 is estimated to account for over 90% of all new cases in Japan and will likely replace all other omicron subvariants by early next month.

Isn’t omicron less severe than delta and more like the flu?

Symptoms caused by omicron are generally considered milder compared with those caused by delta, which raged across Japan in the summer of 2021 and drove up hospitalizations and deaths. Death rates and rates of severe illness for those under the age of 60 are estimated at 0.01% and 0.03%, respectively. Those figures are comparable to those of seasonal flu among people in the same age group. But among those over 60, mortality and severity rates are much higher for COVID than for flu, according to data recently compiled by the health ministry. It should be noted that the omicron data was compiled from patients diagnosed with COVID-19 in January and February, when the original omicron variant was dominant and before BA.2 or BA.4 and BA.5 started spreading in Japan.

Differences in severity and deaths among omicron subvariants are not clear. The World Health Organization said in a report last week that so far, BA.5 seems no different in severity compared with BA.2 and BA.4.

Two and a half years into the pandemic, it’s difficult to judge how pathogenic a particular variant or subvariant is, as people’s immunological backgrounds — determined by their vaccination and infection history — have diversified. Mutations of the coronavirus alone can’t determine how severe the disease’s impact on a population can be, experts say.

What are the typical symptoms of BA.5 and how long do they last?

The symptoms that accompany omicron are somewhat different than those of delta. A Tokyo survey comparing symptoms in August 2021, when delta was dominant, with those in January, when the original omicron subvariant BA.1 was spreading, showed that, on top of fever, a headache and coughing, throat pain was a major symptom reported among omicron patients. Smelling and taste disorders, a symptom seen with previous coronavirus variants, receded during the omicron wave.

Omicron, though highly transmissible, doesn’t lead to symptoms in some individuals. But the percentage of people with no symptoms appears lower for those with BA.4 and BA.5 compared with those with BA.1, said Toshibumi Taniguchi, an infectious disease specialist at Chiba University Hospital, citing a recent report by the French public health authorities.

According to the report, the median illness duration was seven days for BA.4 and BA.5 patients, compared with four days for BA.1 patients. The percentage of people with no symptoms was 3% of the patients for the former and 10.9% for the latter. Common symptoms remained the same: a sense of fatigue, cough, fever, headache and muscle pain. But more people with BA.4/BA.5 complained of a runny nose, throat pain and nausea compared with those infected with BA.1, the study shows.

“It suggests that when people get infected, those with BA.4/5 are more likely to develop symptoms,” Taniguchi told a recent news briefing. “Symptoms also last slightly longer for BA.5 patients.”

Do existing drugs work for BA.5?

Both overseas and Japanese studies point to limited effects of neutralizing antibody treatments on people with BA.5. Neutralizing antibodies bind to the virus’s spike proteins and thus prevent their entry into host cells. Such treatments have been used on people with light to moderate symptoms of COVID-19 to keep them from becoming severely ill. According to Taniguchi, of several neutralizing antibody drugs out there, only bebtelovimab, a remedy developed by pharmaceutical company Lily that has yet to be approved in Japan, has been shown to be effective against BA.5.

In contrast, three anti-viral drugs that have been approved in Japan — remdesivir, molnupiravir and Paxlovid — appear to be effective against BA.5. Anti-viral medicine works by interrupting the virus’ ability to replicate inside the cells. All of the three reduced virus activity in cultured monkey cells infected with BA.5 and other strains, according to a recent study led by researchers at the University of Tokyo.

Do past infections and vaccines keep people from getting infected with BA.5?

According to a recent pre-print paper that has yet to be peer-reviewed from Weill Cornell Medicine-Qatar, a branch of Cornell University in the Middle Eastern country, infection with a pre-omicron strain of the coronavirus offers limited protection against BA.4 and BA.5. Previous infections with, for example, the alpha or delta variant was only around 15% effective in preventing symptomatic BA.4/BA.5 reinfection and about 28% effective in preventing BA.4/BA.5 reinfection irrespective of symptoms, the study found. On the other hand, a prior BA.1/BA.2 infection was about 80% effective in preventing any reinfection with BA.4/BA.5, and about 76% effective in preventing symptomatic BA.4/BA.5 reinfection.

As for vaccines, current shots are based on the original coronavirus strain first reported in Wuhan, China, from which recent variants have heavily mutated. The current vaccine is therefore not effective enough to keep people from getting infected with omicron subvariants. But the government still recommends people get a third shot, as the vaccines are believed to prevent severe symptoms. The rollout of fourth shots in Japan started in late May and was originally limited to high-risk people and people age 60 or older, but was expanded to include medical and elderly care workers last week.

While over 90% of the elderly population in Japan have received three shots, the overall percentage of people who received a booster shot stood at 62.7% as of Wednesday.

Taniguchi says booster shots have been shown to raise the level of neutralizing antibodies in the body and protect people from getting severely ill against omicron subvariants, including BA.5.

He also said that the booster shot is effective in reducing the “infectious viral load,” or the ability of the infected to transmit the virus to others. By receiving booster shots themselves, parents can better protect their children, especially those under age 5, who are currently not eligible for COVID-19 vaccines in Japan, he said.