Strict lockdowns in New Zealand last year appear to have contributed to a recent outbreak in children of respiratory syncytial virus, or R.S.V., a highly contagious, flulike illness whose symptoms include a runny nose, coughing, sneezing and fever.

Children in New Zealand were mostly stuck indoors amid lockdowns last autumn, which runs from March to May in the Southern Hemisphere. After the country reopened last winter, health officials say, few of them contracted seasonal viruses and infections, probably because they had been underexposed to germs.

In a typical year, New Zealand sees a peak of cases of respiratory infections from June to September. But in 2020, the country experienced the complete absence of an annual winter influenza epidemic, with a 99.9 percent reduction in flu cases and a 98 percent reduction in R.S.V., according to a study published in Nature in February.

This year, however, the same children have been more vulnerable than usual to those same ailments.

Since the start of winter five weeks ago, during which there have been no coronavirus restrictions, childrens wards in New Zealand have seen dozens of patients, many of them infants, battling the sometimes deadly disease, while some elementary schools have reported having as many as half their students absent because of respiratory illnesses.

The country has reported 969 cases of R.S.V. in five weeks, compared with an average of 1,743 cases over the entire 29-week winter season in the five years before the pandemic, according to New Zealands Institute of Environmental Science and Research.

The recent surge has yet to reach a plateau, said Dr. Sue Huang, a virologist at the Institute of Environmental Science and Research and the lead author of the Nature study.

The exponential increase is very sharp, she said in a statement. The absence of R.S.V. last winter meant there is a young cohort of children from last year, plus a new cohort this year, who have not been exposed to the seasonal virus.

Doctors around the world have warned of the risk of immunity debt, when a decline in the number of viral and bacterial infections during lockdowns is followed by more sickness once restrictions are lifted.

In a paper published in May in the journal Infectious Diseases Now, a team of French medical researchers suggested that less exposure to microbial agents could create a lack of immune stimulation for susceptible people, particularly in children. The longer these periods of viral or bacterial low-exposure are, the greater the likelihood of future epidemics, they wrote.

New Zealand closed its borders early in the pandemic, unveiling stringent lockdown measures that were lifted last April and May and have allowed the country to all but eliminate coronavirus transmission. No community cases have been reported for more than four months.

In other developments around the world:

Hundreds of Cubans took to the streets in cities around the country on Sunday to protest food and medicine shortages brought on by the pandemic, in a remarkable eruption of discontent not seen in nearly 30 years. Hundreds of people marched through San Antonio de los Baos, southwest of Havana, with videos streaming live on Facebook for nearly an hour before they suddenly disappeared. As the afternoon wore on, other videos appeared from demonstrations elsewhere, including Palma Soriano, in the countrys southeast. Hundreds of people also gathered in Havana, where a heavy police presence preceded their arrival.

Spain is facing a further hit to its summer tourism season after Germany classified the entire country as a risk area. As of Sunday, travelers entering Germany from anywhere in Spain, including its Balearic and Canary archipelagoes, will have to provide a negative coronavirus test or proof of vaccination or recovery to avoid quarantine. The Italian government also warned on Saturday that it was considering greater restrictions on travelers from Spain as well as Portugal. Both countries have been grappling with a surge in coronavirus cases fueled by the highly contagious Delta variant, particularly among unvaccinated young people.

Raphael Minder contributed from Madrid.

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New Zealand, where Covid-19 is dormant, fights another respiratory virus, and other news from around the world. - The New York Times