Its a question that has haunted scientists since the pandemic began: Does everyone infected with the virus produce antibodies and if so, how long do they last?

Not very long, suggests a new study published Thursday in Nature Medicine. Antibodies protective proteins made in response to an infection may last only two to three months, especially in people who never showed symptoms while they were infected.

The conclusion does not necessarily mean that these people can be infected a second time, several experts cautioned. Even low levels of powerful neutralizing antibodies may still be protective, as are the immune systems T cells and B cells.

But the results offer a strong note of caution against the idea of immunity certificates for people who have recovered from the illness, the authors suggested.

Antibodies to other coronaviruses, including those that cause SARS and MERS, are thought to last about a year. Scientists had hoped that antibodies to the new virus might last at least as long.

Several studies have now shown that most people who are visibly ill with Covid-19 develop antibodies to the virus, although it has been unclear how long those antibodies last. The new study is the first to characterize the immune response in asymptomatic people.

The researchers compared 37 asymptomatic people to an equal number who had symptoms in the Wanzhou District of China. The investigators found that asymptomatic people mount a weaker response to the virus than those who develop symptoms.

Antibodies fell to levels below the threshold for a seropositive diagnosis in 40 percent of asymptomatic people, compared with just 13 percent of symptomatic people.

The sample size is small, however, and the researchers did not take into account protection offered by immune cells that may fight the virus on their own or make new antibodies when the virus invades. A few studies have shown that the coronavirus stimulates a robust and protective cellular immune response.

Most people are generally not aware of T cell immunity, and so much of the conversation has focused on antibody levels, said Angela Rasmussen, a virologist at Columbia University.

Apart from T cells, which can kill the virus on encounter, people who have been infected make so-called memory B cells, which can rapidly ramp up antibody production when needed.

If they find the virus again, they remember and start to make antibodies very, very quickly, said Florian Krammer, a virologist at the Icahn School of Medicine at Mount Sinai, who has led several studies of antibodies to the coronavirus.

In the new study, antibodies to one viral protein dropped below detectable levels. But a second set of antibodies targeting the so-called spike protein of the coronavirus needed to neutralize the virus and prevent reinfection were still present.

In fact, these antibodies seemed to show a smaller decline in asymptomatic people than in symptomatic people. The neutralizing antibody is what matters, and that tells a very different story, Dr. Krammer said.

Updated June 16, 2020

The steroid, dexamethasone, is the first treatment shown to reduce mortality in severely ill patients, according to scientists in Britain. The drug appears to reduce inflammation caused by the immune system, protecting the tissues. In the study, dexamethasone reduced deaths of patients on ventilators by one-third, and deaths of patients on oxygen by one-fifth.

The coronavirus emergency relief package gives many American workers paid leave if they need to take time off because of the virus. It gives qualified workers two weeks of paid sick leave if they are ill, quarantined or seeking diagnosis or preventive care for coronavirus, or if they are caring for sick family members. It gives 12 weeks of paid leave to people caring for children whose schools are closed or whose child care provider is unavailable because of the coronavirus. It is the first time the United States has had widespread federally mandated paid leave, and includes people who dont typically get such benefits, like part-time and gig economy workers. But the measure excludes at least half of private-sector workers, including those at the countrys largest employers, and gives small employers significant leeway to deny leave.

So far, the evidence seems to show it does. A widely cited paper published in April suggests that people are most infectious about two days before the onset of coronavirus symptoms and estimated that 44 percent of new infections were a result of transmission from people who were not yet showing symptoms. Recently, a top expert at the World Health Organization stated that transmission of the coronavirus by people who did not have symptoms was very rare, but she later walked back that statement.

Touching contaminated objects and then infecting ourselves with the germs is not typically how the virus spreads. But it can happen. A number of studies of flu, rhinovirus, coronavirus and other microbes have shown that respiratory illnesses, including the new coronavirus, can spread by touching contaminated surfaces, particularly in places like day care centers, offices and hospitals. But a long chain of events has to happen for the disease to spread that way. The best way to protect yourself from coronavirus whether its surface transmission or close human contact is still social distancing, washing your hands, not touching your face and wearing masks.

A study by European scientists is the first to document a strong statistical link between genetic variations and Covid-19, the illness caused by the coronavirus. Having Type A blood was linked to a 50 percent increase in the likelihood that a patient would need to get oxygen or to go on a ventilator, according to the new study.

The unemployment rate fell to 13.3 percent in May, the Labor Department said on June 5, an unexpected improvement in the nations job market as hiring rebounded faster than economists expected. Economists had forecast the unemployment rate to increase to as much as 20 percent, after it hit 14.7 percent in April, which was the highest since the government began keeping official statistics after World War II. But the unemployment rate dipped instead, with employers adding 2.5 million jobs, after more than 20 million jobs were lost in April.

Mass protests against police brutality that have brought thousands of people onto the streets in cities across America are raising the specter of new coronavirus outbreaks, prompting political leaders, physicians and public health experts to warn that the crowds could cause a surge in cases. While many political leaders affirmed the right of protesters to express themselves, they urged the demonstrators to wear face masks and maintain social distancing, both to protect themselves and to prevent further community spread of the virus. Some infectious disease experts were reassured by the fact that the protests were held outdoors, saying the open air settings could mitigate the risk of transmission.

States are reopening bit by bit. This means that more public spaces are available for use and more and more businesses are being allowed to open again. The federal government is largely leaving the decision up to states, and some state leaders are leaving the decision up to local authorities. Even if you arent being told to stay at home, its still a good idea to limit trips outside and your interaction with other people.

Common symptoms include fever, a dry cough, fatigue and difficulty breathing or shortness of breath. Some of these symptoms overlap with those of the flu, making detection difficult, but runny noses and stuffy sinuses are less common. The C.D.C. has also added chills, muscle pain, sore throat, headache and a new loss of the sense of taste or smell as symptoms to look out for. Most people fall ill five to seven days after exposure, but symptoms may appear in as few as two days or as many as 14 days.

If air travel is unavoidable, there are some steps you can take to protect yourself. Most important: Wash your hands often, and stop touching your face. If possible, choose a window seat. A study from Emory University found that during flu season, the safest place to sit on a plane is by a window, as people sitting in window seats had less contact with potentially sick people. Disinfect hard surfaces. When you get to your seat and your hands are clean, use disinfecting wipes to clean the hard surfaces at your seat like the head and arm rest, the seatbelt buckle, the remote, screen, seat back pocket and the tray table. If the seat is hard and nonporous or leather or pleather, you can wipe that down, too. (Using wipes on upholstered seats could lead to a wet seat and spreading of germs rather than killing them.)

The C.D.C. has recommended that all Americans wear cloth masks if they go out in public. This is a shift in federal guidance reflecting new concerns that the coronavirus is being spread by infected people who have no symptoms. Until now, the C.D.C., like the W.H.O., has advised that ordinary people dont need to wear masks unless they are sick and coughing. Part of the reason was to preserve medical-grade masks for health care workers who desperately need them at a time when they are in continuously short supply. Masks dont replace hand washing and social distancing.

If youve been exposed to the coronavirus or think you have, and have a fever or symptoms like a cough or difficulty breathing, call a doctor. They should give you advice on whether you should be tested, how to get tested, and how to seek medical treatment without potentially infecting or exposing others.

A second paper, published on Thursday in the journal Nature, suggests that even low levels of antibodies might be enough to thwart the virus. It does appear that even low levels of certain antibodies have potent neutralizing capability, said Dr. Rasmussen, the Columbia University virologist. Low antibody titers dont necessarily determine whether a patient will be protected from reinfection.

Between 20 and 50 percent of those infected may never show outward signs of the illness. The new study from China, which tracked people over time to confirm that they never developed symptoms, put that number at 20 percent.

About a third of the asymptomatic people had the ground-glass opacities characteristic of Covid-19 and abnormalities in the lungs and in cell types, however.

The study also found that asymptomatic people shed virus when infected, and did so for longer than those who had symptoms. That finding is interesting because it actually might suggest that these asymptomatic patients are indeed capable of transmitting virus, Dr. Rasmussen said.

But she and other experts noted that its unclear whether the virus shed by asymptomatic people is capable of infecting others. It is important to know if they are shedding infectious virus, or just remnants of the virus, said Akiko Iwasaki, a viral immunologist at Yale University.

Dr. Iwasaki was more concerned than the other experts about the two new studies.

These reports highlight the need to develop strong vaccines, because immunity that develops naturally during infection is suboptimal and short-lived in most people, she said. We cannot rely on natural infection to achieve herd immunity.

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You May Have Antibodies After Coronavirus Infection. But Not for Long. - The New York Times