The 5 most popular scientific papers of April 2021 in the Nature Index journals

Vaccine efficacy against new COVID-19 strains and the possibility of learning during lucid dreaming feature in these widely discussed studies.

  • Bec Crew

A nine-day-old human-monkey chimera embryo was the subject of the most-discussed paper of April.

The 5 most popular scientific papers of April 2021 in the Nature Index journals

Vaccine efficacy against new COVID-19 strains and the possibility of learning during lucid dreaming feature in these widely discussed studies.

20 July 2021

Bec Crew


A nine-day-old human-monkey chimera embryo was the subject of the most-discussed paper of April.

April’s most talked-about natural-sciences papers explore issues related to COVID-19 immunity and a controversial experiment that gave rise to human-monkey chimeric embryos.

Also featured are studies investigating a person’s ability to communicate during lucid dreaming and the shrinking of the world’s glaciers.

Here is an Altmetric ranking of April’s most popular papers in the natural sciences, published in the 82 high-quality journals tracked by the Nature Index.

1. “Chimeric contribution of human extended pluripotent stem cells to monkey embryos ex vivo”


This study describes how researchers in China, the United States and Spain injected human stem cells into primate embryos and managed to grow human-monkey chimeric embryos for up to 20 days in a culture.

The research, led by a team at the State Key Laboratory of Primate Biomedical Research at Kunming University of Science and Technology in China and the Salk Institute for Biological Studies in San Diego, California, offers potential new models of human biology and disease, which could drive discoveries in developmental biology and evolution.

"As we are unable to conduct certain types of experiments in humans, it is essential that we have better models to more accurately study and understand human biology and disease," said senior author Juan Carlos Izpisua Belmonte, a developmental biologist at the Salk Institute.

There is also the possibility that these chimeras, if they can be developed further, could be a source of organs and tissues for transplantation, the researchers write in the paper.

The study raises major ethical and legal concerns, ethics researchers Julian Savulescu from the University of Melbourne, Australia and César Palacios-González from the University of Oxford, UK write for The Conversation, “because these creatures could possess an ambiguous moral status: somewhere between that of humans, which we don’t tend to experiment upon, and animals, which we do.”

The study drew a great deal of attention on social media and from the media, according to Altmetric. It has been covered by more than 400 online news outlets to date and has reached an estimated audience on Twitter of more than 1.6 million.

2. “Prior SARS-CoV-2 infection rescues B and T cell responses to variants after first vaccine dose”


A single dose of the Pfizer/BioNTech vaccine boosts protection against variants of SARS-CoV-2, but only in those who have previously been infected by COVID-19, this study led by researchers from Imperial College London and Queen Mary University of London, UK, suggests.

The study, which involved researchers from hospitals in the UK, investigated the immune responses of local healthcare workers after they had received their first dose of the vaccine. Blood samples were analysed for immunity against the original strain of SARS-CoV-2 and the Alpha (B.1.1.7) and Beta (B.1.351) variants.

Those who had previously experienced a mild or asymptomatic COVID-19 infection were found to have much higher protection against the variants. Those who had not been previously infected had a weaker immune response, professor of immunology and respiratory medicine at Imperial College London.

The paper has been covered by more than 100 news outlets. It’s reached an audience on Twitter of more than 7.6 million.

3. “Multiple SARS-CoV-2 variants escape neutralization by vaccine-induced humoral immunity”


This study, published in Cell, suggests that antibodies raised by the two messenger RNA vaccines available, manufactured by Pfizer and Moderna, may not be as effective at neutralizing certain variants of SARS-CoV-2, the virus that causes COVID-19.

Using blood samples from 99 subjects who had received their first and second vaccinations, the team, led by researchers at the Massachusetts General Hospital (MGH) in Boston and Harvard University in Cambridge, Massachusetts, tested the effectiveness of antibodies to neutralise the spike protein – the mechanism that allows the virus to penetrate the cells – of 10 globally circulating strains of SARS-CoV-2.

They found that the Beta variant (B.1.351) and Gamma variant (P.1) may be better at evading the immune response triggered by messenger RNA vaccines.

However, the researchers stressed that the results do not undermine the importance of vaccination. "Our findings don't necessarily mean that vaccines won't prevent COVID, only that the antibody portion of the immune response may have trouble recognizing some of these new variants,” said senior author Alejandro Balazs, group leader at the MGH’s Ragon Institute.

Anne Goffard, a virologist at the Université de Lille in France who was not involved in the study, emphasized that the study was done in vitro, “meaning that conclusions cannot be drawn as to the clinical repercussions of these results, which were obtained from cultured cells", she wrote for The Conversation,

Since the publication of this study, various others have tested the efficacy of these vaccines against the various strains. A study published in Nature in July, for example, found that two doses of the Pfizer vaccine are highly effective against the Alpha, Beta and Delta variants.

The Cell study has been covered by 86 news outlets so far, and has reached an estimated audience on Twitter more than 12 million.

4. “Real-time dialogue between experimenters and dreamers during REM sleep”

Current Biology

People who are asleep and experiencing a lucid dream (where they are aware of the fact that they are dreaming) can answer questions in real-time using electrophysiological signals, according to this study.

The research team, involving scientists from the United States, Italy, France, Germany and the Netherlands, recruited 36 volunteers. The volunteers were involved in several experiments in one of four independent labs located in Europe and the US.

In Germany, for example, the lucid-dreaming volunteers were asked maths questions via Morse code. By incorporating the question into a dream scenario, for example, using a bus ticket machine, the volunteers would communicate their answer using pre-arranged facial or eye movements.

In the French, American and Dutch labs, the volunteers were asked questions using spoken language, for example, ‘What is three plus one?’ and ‘Do you like chocolate?’, study co-author Kristoffer Appel, a sleep and dream researcher at the University of Osnabrück and the Institute of Sleep and Dream Technologies in Hamburg, Germany, told Asher Jones at The Scientist.

Six volunteers were able to answer the question correctly on 29 different occasions, the team reports.

The results, which will need to be replicated in future studies to better understand the results, point to the possibility of learning during sleep, for instance, memory tasks, psychotherapy or creative problem-solving, says Osnabrück.

The paper has been covered by 200 online news outlets to date and has reached an estimated audience on Twitter of almost 4 million.

5. “Accelerated global glacier mass loss in the early twenty-first century”


The world’s glaciers are becoming thinner and losing mass at an accelerated pace, according to this study led by researchers from the Swiss Federal Institute of Technology Zurich (ETH Zurich) in Switzerland and the University of Toulouse in France.

The study draws on what the team describes as largely untapped satellite archives to chart surface elevation changes over all glaciers on Earth – around 220,000 in total – defined as being distinct from the Greenland and Antarctic ice sheets.

The results show that between 2000 and 2019, these glaciers lost a total of 267 gigatonnes (one gigatonne equals one billion tonnes) of ice per year, on average. Between 2000 and 2004, they lost 227 gigatonnes of ice per year, but between 2015 and 2019, they lost 298 gigatonnes annually.

According to ETH Zurich, the findings will be included in the next Assessment Report of the United Nations Intergovernmental Panel on Climate Change (IPCC), expected to be published later this year.

"Our findings are important on a political level. The world really needs to act now to prevent the worst-case climate change scenario," says co-author Daniel Farinotti, head of the glaciology group at ETH Zurich.

The paper has been mentioned by more than 200 news outlets to date and has reached an audience on Twitter of more than 5 million, according to Altmetric.