NEW YORK - A new study has an encouraging message for Americans who shy away from Covid-19 shots because of worries about side effects: the chills, fatigue, headache and malaise that can follow vaccination may be signs of a vigorous immune response.

People who had those side effects after the second dose of a Covid-19 vaccine had more antibodies against the coronavirus at one month and six months after the shot, compared with those who did not have symptoms, according to the new study.

Increases in skin temperature and heart rate also signalled higher antibody levels.

“We know that vaccine uptake can be challenging, and in some cases, it can be so because some people have strong reactions to the vaccine,” said Dr Aric Prather, a clinical psychologist at the University of California, San Francisco, who led the study.

“My hope is that this actually helps assuage some of those concerns,” said Dr Prather. “In fact, those symptoms, though they may be unpleasant, may actually be working for you.”

The study has not been reviewed for publication in a scientific journal. But several experts said its results were consistent with those from other research.

The relative increase in antibody levels among those who experienced side effects was small and does not mean that people without symptoms do not muster a strong immune response, experts said.

“Lack of side effects should not be taken as a sign that the vaccine’s not working,” said Dr Alessandro Sette, co-director of the La Jolla Institute of Immunology’s Centre for Vaccine Innovation.

An earlier study found that 98 per cent of people who felt no ill effects still produced copious amounts of antibodies, compared with 99 per cent of those who had localised symptoms or worse, Dr Sette said.

Still, the new results suggest that people who have a rough time after vaccination are likely to be well protected from the virus.

“If you’re feeling crappy, then odds are that you’re probably mounting a pretty reasonable immune response,” said Dr Deepta Bhattacharya, an immunologist at the University of Arizona.

In the new study, Dr Prather and his colleagues tracked symptoms among 363 participants who received the Pfizer or Moderna Covid-19 vaccines.

Those who had side effects produced nearly double the levels of antibodies as those who reported no symptoms, the researchers found. NYTIMES