CORONAVIRUS cases in India are now falling fast, but around the world several other countries are struggling to contain rising numbers of infections due to the variant first detected in India.

In the UK, case numbers due to this variant – previously called B.1.617.2 but now named “delta” – are rising exponentially, sparking fears of a third wave and threatening plans to end lockdown restrictions in England later this month.

In China, parts of the city of Guangzhou, which has a population of 15 million, have been locked down and people banned from leaving without a negative covid-19 test. Meanwhile, Vietnam – one of the few countries that has prevented a major coronavirus outbreak – is trying to contain a cluster of cases that it  says are due to “a hybrid” of delta and the alpha (or B.1.1.7) variant that originated in the UK.

There is growing evidence that delta is even more transmissible than alpha. In the UK, it has rapidly become the most common variant, overtaking alpha around mid May.

Higher transmissibility makes it harder to prevent the spread of a variant. Restrictions that contained the spread of older variants may no longer be enough to contain delta. Although vaccines appear to be only slightly less effective at preventing symptomatic cases caused by delta, higher transmissibility also raises the herd immunity threshold, adding to the difficulties of preventing spread.

Read more: How effective are the different vaccines against covid-19 variants?

This means that even in the UK, one of the countries with the highest vaccination rates, a more transmissible variant could still cause a major third wave of cases, hospitalisations and deaths if the spread of the variant isn’t controlled. According to an update by the UK’s Scientific Advisory Group for Emergencies (SAGE) on 13 May, if delta is 50 per cent more transmissible, continuing to relax restrictions “would lead to a substantial resurgence of hospitalisations (similar to, or larger than, previous peaks)”. In countries with lower vaccination rates, the threat is even greater.

France has imposed restrictions on travel from the UK in an attempt to contain the spread of delta, but it may have left it too late. The variant has already become the dominant one in France, according to sequence data. The same is true for Bangladesh, Japan and Singapore, while Australia and New Zealand are trying to quash outbreaks of the variant.

In most other countries for which data is available, the proportion of cases caused by delta is still low, but this could change fast if the variant is as transmissible as feared. South Africa is starting to see a major increase in cases, for instance.

As for the new “hybrid variant” detected in Vietnam, which the country’s health minister has said is even more contagious than the delta and alpha variants it is made of, very little is actually known. It isn’t clear, for instance, whether this is essentially delta with some extra mutations resembling those of alpha, or whether the two variants have really recombined.