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

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

The cause of this sharp decline in cases remains a mystery. As in other countries that, at some point, have experienced surprising falls in the cases of Covid-19 – such as India, Pakistan and some parts of Brazil – epidemiologists and virologists are putting together different explanations as to why the outbreak in South Africa does not follow the standards set elsewhere.

This ranges from important population groups that achieve sufficient levels of immunity to delay transmission to people who follow more rules of social detachment, such as wearing masks and voluntarily reducing contacts, when deaths increased before the decline.

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

After more than a year of the pandemic, scientists still do not know about how the coronavirus moves in society, often creating waves of infections whose peaks coincide with crowded hospitals and a large number of deaths. Meanwhile, during depressions, life in some places can seem almost normal again.

Bridging the gaps in the global understanding of the virus can have important implications for public health decisions. This includes how to calibrate government interventions, such as roadblocks, where it is best to target vaccines and when a country or region has achieved collective immunity – or, if possible, due to the emergence of new strains of coronavirus.

A complicating factor in South Africa, as in some other countries, is that researchers do not know the true number of victims of the virus in the population. Due to limited testing capabilities and asymptomatic infections, there is 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, reinfected people who recovered from a previous Covid-19 attack.

In contrast to the drop in cases that much of Europe experienced last summer, the current drop in infections in South Africa has not followed 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 popular beaches, tightened the night curfew and banned large social gatherings and the sale of alcoholic beverages. A national mask mandate has been in effect since April 2020.

Indoor meals with limited capacity, however, were allowed 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 like Johannesburg had already traveled to see the family in provinces where Covid-19’s number of cases was double the number recorded in July, during the first wave. The workers’ return home in early January, often huddled in minibus taxis that are a common means 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 parts of the population have reached a level of immunity that has made it difficult for the virus to pass between different groups, said Jinal Bhiman, a leading NICD medical scientist.

Only about 1.5 million South Africans, about 2.5% of the population, tested positive for Covid-19. But it is clear that the actual level of infection was much higher. Since cases began to rise in May, the country has recorded more than 145,000 excess 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 average age is a decade below the United States – has died of the disease in the past 10 months.

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

Experts also warned that blood donors are not representative of the general population, as shown by the recent resurgence of infections in the Brazilian city of Manaus, where a study of antibodies to blood donated last year 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 work contacts, in conducting and eventually reducing localized outbreaks. “Highly sociable people are infected first, and the virus moves through these networks,” said Dr. Moultrie. When enough people in these networks become immune, transmission slows down.

Researchers around the world are also studying the impact of voluntary changes in behavior, which can anticipate and strengthen government 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 are already decreasing can further accelerate the fall.

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

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

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

Write to Gabriele Steinhauser at [email protected]