This brief backgrounder explains the currently known facts on the delta variant and the lambda variant. [UPDATED SEPTEMBER 3, 2021]

All viruses mutate over time, and it is the nature of an mRna virus like the coronavirus to mutate. As a virus spreads across larger geographic spaces for longer periods of time, it is unsurprising that varying mutations occur. This is what has happened with the coronavirus in the last 18 months. To date, there are nearly 200 million confirmed cases of the coronavirus spread across every nation in the world. The actual number of coronavirus cases is likely much larger than what is confirmed, as many poorer nations still lack the healthcare infrastructure to quickly diagnose their populations.

The first version of the coronavirus was detected in China, but new mutations (called variants in the popular media) began to develop over time. In September 2020, Britain detected a new mutation of the virus. Within a few months, that variant quickly became the most dominant form of the virus in different countries worldwide. Variants were also identified in Brazil, South Africa, and California in 2020.

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Initially, these variants were known by the names given to them by scientists in laboratories. The British variant was called B.1.1.7. Some scientists referred to the variants based upon where they were first discovered, such as the South African variant. As more variants were identified, the world health community adapted the Greek alphabet for naming the variants. The first coronavirus variant found in the United Kingdom became known as Alpha after the first letter in the Greek alphabet.

The mutations of the variants create a level of unknown that presents new risks to the scientific and medical communities. Vaccines and treatments are based upon what is known about the coronavirus. When the virus mutates, new factors must be studied to determine if the treatment and vaccines are still adequate in combatting the latest form of the virus.

Delta Variant

The Delta variant is the mutated form of the coronavirus that was first identified in India. It is now the dominant strain of the coronavirus in the US, accounting for more than half of all active confirmed coronavirus cases in the first week of July. Delta spread quickly around the world. Delta was first identified in the US in March, and its percentage among the active case population nearly doubled every week since that time.

Delta is spreading quickly not only in the US but around the world. Its presence is confirmed in more than 100 countries.

Delta, like all variants of the coronavirus, spread easiest among unvaccinated populations. Counties and areas of the country with lower vaccination rates are seeing new surges of the pandemic due to the delta variant.

The World Health Organization (WHO) announced that delta is the most transmissible form of the coronavirus seen so far. Individuals infected with delta infect more people than those infected with the alpha strain of the coronavirus. When a nation’s population is largely unvaccinated, delta presents unique and extreme risks. That reality was demonstrated by the coronavirus wave that hit India in May and peaked out with more than 400,000 new cases per day. Hospitals ran out of oxygen for treating the sick. Cemeteries ran out of daylight hours to bury the dead.

Early studies suggest delta is 40-60% more infection than the first strain of the coronavirus. Another study indicated that delta is more transmissible and more deadly, increasing the likelihood of hospitalization and health risks for those who contract the virus. A study out of Scotland found individuals who contracted delta were twice as likely to need hospitalization.

So far, the vaccines are still effective against the variant, although effectiveness does appear to be reduced. A study out of Israel suggested that the 95% efficacy of the Pfizer vaccine was reduced to 64% against delta. Vaccinated individuals who contracted delta still experienced less severe health impact than unvaccinated individuals.


Many hoped the presence of the vaccines would reduce the risks of coronavirus infections, serious illness and death. That proved true for a time but then the delta variant arrived. A study in mid-August based upon data from seven states found one in five new cases included people who were fully vaccinated. These so-called breakthrough infections also accounted for a higher percentage of hospitalizations than originally thought. Breakthrough infections accounted for 12-24% of covid related hospitalizations.

The vaccines remain the strongest combatant to the coronavirus but the delta variant appears to be changing the math.

Mu Variant

The World Health Organization (WHO) has listed the Mu variant as a new variant of interest. It is the fifth coronavirus variant of interest. . The Mu variant was first identified in Colombia in January and now accounts for 39% of the recorded cases in that country. The Mu variant appears to have the ability to evade immunity provided by vaccines. The Mu variant has been found in 40 countries to date.

The US has identified 2,000 Mu cases as of September 1. The bulk of these cases were in California, Texas, New York, and Florida.

Lambda Variant

Although many have heard of the delta variant, fewer are aware of the lambda variant, which also presents a serious health threat in different parts of the world. Lambda is the dominant strain in Peru which currently has the highest fatality and death per capita rate globally. In Peru, for every 100,000 people in the population, 596 have died of the coronavirus. That is near twice the rate of deaths in Hungary which has the second-highest coronavirus mortality rate.

Lambda was first seen in Peru in December 2020. Today it accounts for 80% of Peru’s confirmed coronavirus cases.

The World Health Organization (WHO) says lambda is now present in 29 countries

spread of lambda
Source: Global Initiative on Sharing All Influenza Data (GISAID), Peruvian National Health Institute • As of July 27, 2021
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JB Shreve is the author of "How the World Ends: Understanding the Growing Chaos." He has been the host of the End of History podcast since 2012. He has degrees in International Relations and Middle East Studies. His other books include the Intelligence Brief Series. Regular posts and updates from JB Shreve are available at