Falling Covid-19 cases in South Africa adds to questions about waves of infections

Since mid-January, Covid-19-confirmed infections in South Africa have fallen from a record nearly 22,000 a day to about 1,000, without a large-scale vaccination campaign or strict blockade. Less than 5% of Covid-19 tests find traces of the virus, a sign that health agencies are missing fewer cases. The government has removed most of the remaining virus restrictions for the country of 60 million people.

The cause of this sharp decline in cases remains somewhat mysterious. As in other countries that have at some point experienced sharp falls in the Covid-19 cases — such as India, Pakistan, and parts of Brazil — epidemiologists and virologists are gathering different explanations for why the outbreak in Covid-19 South Africa does not follow the patterns elsewhere.

These range from large population groups that achieve sufficient levels of immunity to slow down transmission, to people who are closer to the rules of social distancing, such as wearing masks and voluntarily reducing contacts, when deaths increased earlier. of the decline.

“Anyone who professes certainty [about why infections started dropping] lies, “said Harry Moultrie, a senior medical epidemiologist at the National Institute of Communicable Diseases of South Africa or NICD.” There is so much uncertainty in all of this. “

In addition to a year of the pandemic, there are still many scientists who do not know about how the coronavirus moves through society, often creating waves of infections whose peaks coincide with overwhelmed hospitals and a large number of deaths. Meanwhile, during drinking troughs, life in some places can return to near normal.

Filling gaps in global understanding of the virus could have important implications for public health decisions. These include how to calibrate government interventions, such as blockades, where it is best to target vaccines, and when a country or region has achieved herd immunity, or if possible, given the emergence of new coronavirus strains.

A complicated factor in South Africa, as in some other countries, is that researchers do not know the true toll of the virus in the population. Due to limited testing capabilities and asymptomatic infections, there are no definitive data on how many people have recovered from Covid-19 and can now be immune.

Virologists continue to study the coronavirus variant, known as B. 1,351, which fueled the latest wave of infections here. The strain appears to make some existing vaccines less effective, and in some cases has reinfected people who had recovered from a previous Covid-19 attack.

In contrast to the fall in cases experienced by much of Europe last summer, the current fall in infections in South Africa did not follow a strict government-imposed blockade. At the turn of the year, which coincides with the main summer holidays in the southern hemisphere, the government closed the popular beaches, tightened the night curfew and banned large social gatherings and the sale of alcohol. A national mask mandate has been established since April 2020.

However, food was allowed indoors with limited capacity and many families gathered for Christmas and New Year. Most of the restrictions came only after tens of thousands of South Africans working in economic centers such as Johannesburg had already traveled to see the family in provinces where the number of Covid-19 cases was double the records set in July. , during the first wave. The return of workers home in early January, often stacked on shared minibuses that are a means of public transportation in South Africa, created the perfect conditions for the virus to spread.

The simplest explanation for the sudden mid-January drop in cases is that sections of the population had reached a level of immunity that made it difficult for the virus to jump between different groups, said Jinal Bhiman, the NICD’s chief medical scientist.

Only about 1.5 million South Africans, about 2.5% of the population, have tested positive for Covid-19. But it is clear that the real level of infection has been much higher. Since cases first began to rise in May, the country has recorded more than 145,000 excess deaths, of which 85% to 95% are probably due to Covid-19, according to the Research Center. Medical of South Africa. This means that approximately one in 500 people in South Africa, where the average age is a decade below in the US, has died from the disease in the last ten months.

South African researchers, after testing the blood of 4,858 donors to detect antibodies in January, estimated that in the two most affected provinces more than half of people between the ages of 15 and 69 had already had Covid-19. But immunity levels are unlikely to be equally high in other parts of the country.

Experts have also warned that blood donors are not representative of the general population, as evidenced by the recent resurgence of infections in the Brazilian city of Manaus, where a blood antibody study given last year found results. similar.

In the absence of national herd immunity, scientists focus on the role of certain networks, or individuals with many social or work contacts, in driving and ultimately curbing localized outbreaks. “Highly sociable people get infected first and the virus moves through these networks,” Dr. Moultrie said. When there are enough people in these networks who are immunized, the transmission ends.

Researchers around the world are also studying the impact of voluntary changes in behavior, which can anticipate and strengthen government-imposed restrictions. “When rates go up, people change their behavior,” said Saad Omer, director of the Yale Institute for Global Health. Just as a small increase in social contacts can lead to an exponential increase in infections, reducing meetings when infections already decrease can further accelerate the fall.

“Small changes can have massive consequences,” Dr. Omer.

Perhaps the hardest question to answer is what will happen next. Will cases increase again, perhaps thanks to another coronavirus strain, as happened about two months after South Africa ended its first wave of infections in September? Juliet Pulliam, who heads the Center for Excellence in Modeling and Epidemiological Analysis in South Africa, says there is no way to know.

“I don’t think it’s possible to predict with certainty when, or even if, there will be a third wave in South Africa,” he said.

Write to Gabriele Steinhauser to [email protected]