Coronavirus cases are falling globally, but the big question is: why?
A theory for the fall in cases in at least parts of the world is seasonality, as other coronaviruses have historically followed a seasonal pattern, rising in autumn and winter and falling back in spring and summer.
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For journalists, scientists and politicians, “sorry” isn’t always the hardest word. Sometimes, it’s “we don’t know”.
And that’s the position that most are in right now, analysing the seemingly hopeful Covid-19 trends emerging across the globe.
For six consecutive weeks, the number of new Covid-19 cases globally have fallen. According to the World Health Organization, cases fell by 11 per cent this week, and deaths by 20 per cent. It’s the first time since the pandemic began more than a year ago that there has been such a sustained drop.
But the big question is: why?
Rather than “we don’t know”, Dr Paul Sax, a professor of medicine at Harvard, has another option: “gemish”, Yiddish for “a mixture of things”. From improved social distancing to lockdowns, the early impact of vaccinations or emerging herd immunity, there is no single reason for the fall in numbers.
“It could be all of the above explanations, in various proportions, and different in various regions – plus things no one has considered,” he said.
Below, we look at what could be behind the fall in cases – and how the world can sustain it.
Are lockdowns having an impact?
In places that are in lockdown – such as many parts of Europe – the answer seems to be yes. Lockdowns are remarkably effective at stopping transmission, although the obvious question, of what happens when they are lifted, remains.
Professor Martin McKee, professor of European public health at the London School of Hygiene and Tropical Medicine, said that while no-one is totally sure what is driving the fall in cases, it is clear that lockdowns and other “non-pharmaceutical interventions” or NPIs, like face masks, are making a huge difference.
Covid-19 cases reported weekly by WHO Region, and global deaths, as of 21 February 2021
“If you get the R number below one you will get steep declines. Just as you can get exponential growth in the number of cases you can also get exponential declines,” he said.
“The R number has been below one for a couple of weeks pretty much everywhere in Europe apart from Greece, Hungary, Bulgaria and Moldova. As long as restrictions stay in place you will see those numbers fall,” he said.
The WHO added that countries with the most substantial decline had seen peaks over the Christmas, and responded with increased public health and social measures.
“Preliminary analysis shows that the countries which had the largest declines had stringent public health and social measures in place, some of which were also put in place as a response to the new variants of concern,” a spokesman said.
The world is also much better at widespread testing and contact tracing, he added, which has had a substantial impact.
So cases aren’t falling everywhere?
The fall in cases is not uniform around the world, or even within regions, as Professor McKee points out. Badly hit regions like the Americas and Europe are driving the drop, with both seeing declining case loads – albeit at different rates of decline, and from such high peaks that the numbers remain significant.
But according to the WHO the drop is not confined to these regions – infections are dropping in 123 countries.
The exceptions to this rosy outlook are the Eastern Mediterranean and South East Asia, where there was a slight increase in cases of seven and two per cent respectively last week. These numbers are driven largely by Iran, Iraq, India and Indonesia.
Is spring making a difference?
Another theory for the fall in cases in at least parts of the world is seasonality, as other coronaviruses have historically followed a seasonal pattern, rising in autumn and winter and falling back in spring and summer.
The WHO said it is considering this hypothesis “as we have seen declines in many countries in the northern hemisphere”.
Dr Sax isn’t convinced, though. “The problem with this seasonality theory is that the seasons are flipped in the southern hemisphere,” he wrote in a recent blog post. “And didn’t cases surge over the summer in many southern US states?”
Or have we reached ‘herd immunity’?
Another seductive theory is that enough people have been infected to confer some level of population-wide protection, meaning the virus has nowhere to go.
In some areas – notably South Africa, where cases and deaths fell by 25 and 30 per cent last week – this is a possible explanation, experts say. Recent seroprevalence surveys have suggested that up to 63 per cent of people in some regions there have Sars-Cov-2 antibodies after the new variant spread like wildfire around the country.
Importantly, those who either cannot or do not follow infection control advice are the most likely to have already been infected, and therefore immune, scientists said.
However, others suggest that the response to South Africa’s huge second wave could also be behind the drop. Dr Marc Mendelson, an infectious disease doctor at Groote Schuur Hospital in Cape Town, said: “As the situation became more serious, it’s likely to have coincided with more people complying, plus we had increased restrictions on gatherings.”
Globally, though, there is a consensus that herd immunity so far has played a limited role. It is now estimated that 90 per cent of people need to have been either vaccinated or have antibodies from a previous bout of infection to come close to herd immunity.
And there are numerous examples of cities – including Manaus, in Brazil, and Mumbai, India – which have seen a substantial resurgence despite devastating first waves.
The arrival of Covid-19 variants also complicates the picture, as there are some suggestions that some may evade the immune response triggered by earlier infections.
What about vaccines?
Despite impressive vaccine rollouts in some countries, Prof McKee said jabs are unlikely to have had a dramatic impact on reducing infections so far. The exception is Israel, which has outpaced the world in administering doses.
In the UK, where more than 18 million mainly older people have had their first dose, the latest update from the React study – which randomly samples 150,000 people every month – shows infection rates falling across all age groups at a similar rate, “suggesting the downward trends are due to lockdown rather than the impact of vaccination”.
There is, though, some emerging evidence suggesting vaccines may have some impact on preventing transmission. This means that, “while the vaccine rollout is not yet broad enough to explain the case number drop on its own, it might be contributing,” said Dr Sax.
Are we out of the woods?
While we’re still not sure why numbers are falling, this point is clear: we are not out of the woods.
The situation has vastly improved, and vaccine roll-outs across the world are a huge step forward. But Covid-19 is a notoriously unpredictable disease. And despite the toll wreaked so far – more than 112 million infected, and at least 2.5 million deaths – billions remain vulnerable.
“The only truth is that this virus continues to do it’s own sweet thing, and continues to teach us new lessons,” said Dr Mendelson.
The WHO also points out that the world is far from done with the pandemic. In absolute terms the numbers remain high, with 66,000 fatalities and 2.4 million new infections reported last week.
“Are we turning the corner? The problem sometimes with corners is you don’t see what’s around that corner,” Dr Mike Ryan, head of the WHO emergencies programme, said earlier this month.
Or, put another way: “This [virus] is like a floodwater; just because the floodwaters have dropped an inch or two it doesn’t mean the flood’s going to go away because it’s still raining upstream.”
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By Sarah Newey; Jennifer Rigby; Anne Gulland and Jordan Kelly-Linden @ telegraph