U.S. pharmaceutical giant Johnson & Johnson paused its late-stage clinical trials of its COVID-19 vaccine after a participant was diagnosed with an unexplained illness. The pause was first reported Monday by the health care news website Stat, which obtained a document the company sent to outside researchers.  Johnson & Johnson had just launched its wide scale testing of its single-dose experimental vaccine. The trial involves 60,000 volunteers across more than 200 locations in the United States and internationally, including Argentina, Brazil, Chile, Mexico and South Africa.   The company said in a statement that so-called “adverse events” such as illnesses and accidents are an expected part of a clinical study, especially with such a large number of participants.  It also said the hold was a “study pause” and not a “clinical hold” which is imposed by a formal health regulatory agency.  Because it can be administered in a single dose, Johnson & Johnson’s vaccine has significant advantages over the other three potential vaccines, which require two doses.   The single-dose vaccine would not have to be kept frozen in ultracold temperatures, making it easier to utilize in a mass immunization campaign.   The Johnson & Johnson vaccine trial is the second to be put on hold after a volunteer became ill after receiving the vaccine.  U.S.-based drugmaker AstraZeneca halted its late-stage trial of a vaccine developed with the University of Oxford early last month after a volunteer in Britain was diagnosed with transverse myelitis, an inflammatory syndrome that affects the spinal cord and is often sparked by viral infections.  FILE – A worker feeds vials for production of SARS CoV-2 Vaccine for COVID-19 at the SinoVac vaccine factory in Beijing.The late-stage testing of that vaccine has resumed in Britain, Brazil, India and South Africa, but remains on hold in the United States.  Meanwhile, a new study suggests that a person infected with COVID-19 is still vulnerable to the disease. A report published Monday in The Lancet Infectious Diseases medical journal revealed a 25-year-old man in (the western U.S. state of) Nevada first tested positive with COVID-19 back in April, then a second time in June with more severe symptoms that led to him being placed on oxygen.    Researchers say the man was infected with two distinct strains of the novel coronavirus, but could not be sure why the second infection was worse. He may have been exposed to a higher dose of the virus the second time, or the later version was more virulent than the first. This is the first confirmed case of COVID-19 reinfection in the United States, and just the fifth of its kind in the world, with one patient each in Belgium, the Netherlands, Hong Kong and Ecuador. The researchers say the reinfections suggest that previous exposure to the coronavirus “does not necessarily translate to guaranteed total immunity.” The Lancet study appears to support a warning by the World Health Organization against a strategy of pursuing herd immunity to stop the coronavirus pandemic, calling the idea unethical.    At a news briefing in Geneva Monday, WHO Director-General Tedros Adhanom Ghebreyesus said health officials should only try to achieve immunity through vaccination, not through exposing people to the virus.     Herd immunity happens when a population is protected from a virus because a threshold immunity has been reached in that society.     “Never in the history of public health has herd immunity been used as a strategy for responding to an outbreak, let alone a pandemic. It’s scientifically and ethically problematic,” Dr. Tedros said.     The WHO estimates that about 10% of the world has contracted the coronavirus. It is not yet known what percentage rate of infection is needed to achieve herd immunity.   Tedros noted that to obtain herd immunity from measles, about 95% of the population must be vaccinated, while for polio, the threshold is about 80%. 
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