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

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

The cause of this sharp drop in cases remains a little mysterious. As in other countries that have at one point seen surprising declines in Covid-19 cases – such as India, Pakistan and parts of Brazil – epidemiologists and virologists are pulling together different explanations as to why the epidemic in South Africa does not follow models established elsewhere.

These range from large population groups achieving levels of immunity sufficient to slow transmission, to people who more closely adhere to social distancing rules, such as wearing masks and voluntary reduction of contact, when deaths increased before the decline.

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

More than a year after the start of the pandemic, many scientists remain unsure of how the coronavirus is moving through society, often creating waves of infections whose peaks coincide with overwhelmed hospitals and large numbers of people. death. During troughs, during this time, life in some places can become almost normal again.

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

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

Virologists continue to study the variant of the coronavirus, 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 re-infected people who had recovered from a previous episode of Covid-19.

Unlike the crisis in much of Europe last summer, the current decline in infections in South Africa has not followed a strict lockdown imposed by the government. At the end of the year, which straddles the southern hemisphere’s main summer vacation, the government closed popular beaches, tightened the nighttime curfew, and banned large social gatherings and the sale of alcohol. A national mask mandate has been in place since April 2020.

However, limited-capacity dining was allowed everywhere and many families gathered for Christmas and New Years. Most of the restrictions only came after tens of thousands of South Africans working in economic centers. like Johannesburg have already traveled to see their families 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 crammed into shared minibus taxis that are a common form of public transport in South Africa, created perfect conditions for the virus to spread.

The simplest explanation for the sudden drop in cases in mid-January is that sections of the population had achieved a level of immunity that made it more difficult for the virus to pass between different groups, said Jinal Bhiman, senior scientist. medical at NICD.

Only around 1.5 million South Africans, or around 2.5% of the population, have tested positive for Covid-19. But it is clear that the actual level of infection has been much higher. Since cases began to increase in May, the country has recorded more than 145,000 additional deaths, of which 85% to 95% are likely due to Covid-19, according to the South African Medical Research Center. This means that about one in 500 people in South Africa – where the median age is ten years younger than that in the United States – has died from the disease in the past 10 months.

South African researchers, after testing the blood of 4,858 antibody donors in January, estimated that in the two hardest-hit provinces, more than half of people aged 15 to 69 had already had Covid-19 . But immunity levels are unlikely to be high in other parts of the country as well.

Experts have also warned that blood donors are not representative of the overall population, as evidenced by the recent spike in infections in the Brazilian city of Manaus, where an antibody study on donated blood last year has found similar results.

In the absence of national collective immunity, scientists are focusing on the role of certain networks, or individuals with many social or professional contacts, in driving and possibly slowing localized outbreaks. “Very sociable people get infected first, and the virus spreads through these networks,” said Dr Moultrie. When enough people in these networks have become immune, transmission turns off.

Researchers around the world are also studying the impact of voluntary behavior changes, which can anticipate and reinforce 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 slight increase in social contact can lead to an exponential increase in infections, reducing gatherings when infections are already down can further hasten the fall.

“Small changes can have huge consequences,” said Dr Omer.

Perhaps the most difficult question to answer is what will happen next. Will cases rise again, possibly fueled by another strain of coronavirus, as happened about two months after South Africa ended its first wave of infections in September? Juliet Pulliam, who heads the South African Center of Excellence in Epidemiological Modeling and Analysis, says there’s no way to find out.

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

Write to Gabriele Steinhauser at [email protected]