Why global Covid infections have plummeted

A significant fall in global coronavirus cases since the turn of the year has bolstered hopes that countries will be able to contain the pandemic despite concern over the emergence of new variants.

Global infection rates have dropped precipitously since the start of 2021, from more than 5m cases a week to 2.5m in mid-February. The sharpest declines have come in some of the countries that were worst affected in late 2020, including the US, UK, South Africa, Israel and Portugal. Case rates in each of these countries have dropped by more than 50 per cent over the past month. 

“It is always positive to see cases decreasing rather than increasing,” said Antoine Flahault, co-director of the Swiss School of Public Health in Zurich. “What gives me hope is that these countries, like the UK, the US . . . will have a window of opportunity to maintain low levels of [virus] circulation. They have a second chance to take control.”

Yet despite tumbling infection rates in hard-hit countries, global infections have begun to inch up in recent days, led by rises in EU countries including Hungary and France, as well as stubbornly high infection rates in others, notably Brazil. There were 2.6m new cases globally in the week to February 23, a 3 per cent increase over five days.

“Although cases are declining in some of the most impacted countries, that trend is not universal and some places are experiencing a surge,” said Mark Lurie, professor of epidemiology at Brown University in the US. “Trends can reverse, as we have witnessed too many times already with Covid-19.”

Experts say that vaccinations have played a role in the falling number of cases in the US and the UK — which have given jabs to 20 and 28 per cent of their populations, respectively — but it does not alone explain the trend.

The UK has imposed one of the strictest lockdowns anywhere in the world over the past two months, leading to a significant drop in new cases, from an average of 56,000 a day in early January to closer to 13,000 this week.

By contrast, some countries that have remained relatively open, such as France and Brazil, have struggled to flatten their epidemic curves. 

To complicate the picture, the US has steered clear of a national lockdown yet infections have fallen sharply. Epidemiologists attribute the 72 per cent drop in cases since early January in part to disciplined physical distancing and less travel after the Thanksgiving and Christmas holidays.

Case numbers also vary by US state, reflecting different approaches. Democrat-run California and Republican-led Florida had similar levels of positive virus tests in early January. Since then, however, cases have fallen more sharply in California, which imposed restrictions, than in Florida, which remained open.

In South Africa, meanwhile, infectious disease experts believe that very high levels of past infections, especially in poorer black communities, were likely to have created a natural barrier to new infections.

They noted that a population does not need to reach “herd immunity” — the threshold of about 60-70 per cent required to prevent a disease from spreading — for some form of natural immunity to slow the spread. 

New cases in India have fallen steadily since July and remain low, in part because of the apparent absence of new variants, as well as to natural immunity that is thought to be as high as 56 per cent in New Delhi.

In France, like in the UK, natural immunity levels were never high enough to bring down cases significantly on their own.

Tight restrictions in November reduced the spread of infection, but their relaxation has pushed cases higher again, as the more infectious variant that first emerged in the UK has spread. In some French regions, cases are rising to pre-Christmas levels, from 144 per 100,000 population per week in early January to 200 at the end of last week.

France this week imposed partial lockdowns in cities where case numbers have spiked, including Nice and Dunkirk.

There are concerns about a renewed upturn in cases across central Europe, accompanied in some countries by the arrival of the variant that emerged in the UK. The Czech Republic has recorded 968 virus cases per 100,000 people over the past two weeks, the EU’s highest per capita infection rate.

For northern hemisphere countries where infections are falling, some have speculated about the role played by the changing season.

Azra Ghani, chair in infectious disease epidemiology at the MRC Centre for Global Infectious Disease Analysis at Imperial College London, said the significance had been exaggerated. “We’re seeing plenty of evidence to counter the idea of a strong seasonal impact,” she said, noting that Covid-19 also circulated in very hot climates. 

Brazil, for example, is in the throes of summer, and is one of several countries that has struggled to reduce infections. Some parts of Brazil that have been hit hardest, such as the Amazonian state of Manaus, also never get cold. 

One explanation for the stubbornly high number of infections in Brazil is the role played by new variants, which can spread quickly across the population without a strict lockdown in place to contain them. Similarly in the UK, the B.1.1.7 variant spread rapidly before the current lockdown.

Despite recent success in tackling the virus, scientists emphasised that all countries remained vulnerable to surges in new cases.

“Any rapid relaxation of protective measures could produce spikes in infection rates,” said Ted Cohen, professor of infectious disease epidemiology at Yale University. “There are large pools of susceptible individuals still at risk.”