CAPE TOWN, South Africa — The pandemic has made a lot of bad things worse, but South Africa's near-total lack of a flu season this year stands out as a rare positive effect.

The country’s National Institute for Communicable Diseases (NICD) has three laboratories that would normally record more than 1,000 cases of flu between April and August, winter in the Southern Hemisphere. As the 2020 season ends, they have recorded just one.

“It is a totally unprecedented event to not see flu,” said Cheryl Cohen, who leads NICD’s respiratory disease team. While she and other experts in South Africa said some people were certainly staying home and not reporting mild sicknesses, they all agreed that South Africa basically skipped its flu season — and that the coronavirus is to thank.

“The main explanation is that measures against coronavirus are having an impact on flu transmission,” Cohen said.

As Northern Hemisphere countries including the United States ramp up the largest effort to vaccinate hundreds of millions of people against the seasonal flu, Southern Hemisphere countries are providing evidence of how basic protective measures such as mask-wearing and school closures, now instituted to stem the spread of the novel coronavirus, could suppress or even eliminate the flu, which kills millions of people every year.

From New Zealand to Australia, Argentina, Chile and South Africa, reported cases of flu have sharply decreased. The hemisphere’s biggest countries implemented stringent coronavirus measures, partly out of fear that the flu and coronavirus would spread simultaneously, overwhelming hospitals. They put heavy lockdowns in place, banned entry to foreigners and closed schools.

The effectiveness of coronavirus measures in preventing flu transmission has left doctors in South Africa with a riddle: Why did they stop flu in its tracks while South Africa ended up in the top five countries globally for coronavirus cases, which now stand at nearly 600,000?

The answer lies in fundamental differences between the two viruses, said Halima Dawood, head of infectious diseases at Greys Hospital, one of the largest health-care facilities in South Africa’s KwaZulu-Natal Province.

“It seems quite clear that the coronavirus is simply much, much more contagious than the flu,” she said. “This isn’t a fluke — this is proof that simple containment measures, when broadly followed, are effective against influenza transmission, but not enough for coronavirus.”

Of all the measures South Africa put in place, Dawood said the most important against the flu was school closures. Studies have shown that children exhibit the highest rates of infection and illness due to influenza. “I think that was the one that interrupted flu’s whole chain,” she said.

While a general understanding of the coronavirus is still emerging, experts have offered numerous explanations for why it is more infectious than the seasonal flu. Foremost is the extent to which it spreads when those who carry it are asymptomatic. Flu is almost always transmitted through symptoms such as sneezing and coughing.

Many people also carry “background immunities” for influenza, Dawood said. Repeated infections, as well as worldwide annual vaccination campaigns, have given large portions of the global population at least some influenza antibodies, although new strains appear nearly every year.

People also began taking extra health precautions that they otherwise might not have. Flu kills around 12,000 South Africans every year. With coronavirus cases climbing steeply at the beginning of the country’s flu season, many people rushed to clinics to get flu shots, hoping to avoid at least one co-morbidity.

At Sunset Pharmacy in Cape Town’s Sea Point neighborhood, nearly four times as many people were vaccinated than during an average year.

“People were lined up outside the pharmacy down to the corner of the street waiting to get their injections,” said Ellis Henen, who has owned the pharmacy for 56 years. “We had to prevent people from coming into the store” because it was too crowded.

While sales of flu medicine have been down, sales of vitamins and other immune boosters have gone up.

Last week, South African President Cyril Ramaphosa announced that the whole country would shift to a reduced, “level 2” lockdown, allowing for all domestic travel to be permitted and a ban on alcohol and tobacco sales to be lifted. Mask-wearing and social distancing protocols in public spaces are still mandated, but Ramaphosa said South Africa was “past its peak” in coronavirus cases.

The continued implementation of personal protective measures may be so effective against flu transmission that entire strains of the virus that relied on incubation in the Southern Hemisphere’s winter before being passed back north might be eliminated for good, said Wolfgang Preiser, head of medical virology at Stellenbosch University and Tygerberg National Health Laboratory Service in Cape Town.

“Maybe if we are lucky, one or two of these [flu strains] won’t make it because they have had no one to infect for the past six or eight months,” he said, adding that such an outcome wouldn’t be confirmed until exhaustive studies had been done. “If it works out like this, it would be a very beautiful positive side effect of covid, one of the few good things to come of it.”

The main threat to that possibility is reintroduction from the Northern Hemisphere, where many countries, and parts of the United States in particular, have not required mask-wearing and school closures.

“Our only threat now will be in opening up the borders,” Dawood said. “We’ve all but gotten rid of flu here, but the same isn’t guaranteed in the rest of the world.”

Bearak reported from Nairobi.