Some immune-compromised Americans will be eligible for a fourth dose this week. 

A third dose for people with compromised immune systems was recommended by the CDC on Aug. 13. The CDC recently shortened the window of Pfizer and Moderna booster shots from six months to five months. This means immune-compromised people who got a third shot in mid-August will be eligible for a fourth dose in mid-January.

People with immune compromised conditions make up about 3% of the U.S. population.

-ABC News' Sony Salzman, Arielle Mitropoulous

Uganda reopened its schools to students on Monday after nearly two years, ending the world's longest school closure due to the coronavirus pandemic.

Schools across the East African nation of 44 million people have been fully or partially closed since March 2020, when the pandemic began. The closures affected more than 10 million learners, according to data from the United Nations Educational, Scientific and Cultural Organization (UNESCO).

Save the Children welcomed the reopening of Uganda's schools but warned that "lost learning may lead to high dropout rates in the coming weeks without urgent action."

The London-based charity revealed in a report last November that up to one in five children in low-income countries, including Uganda, had dropped out of school due to rising poverty, child marriage and child labor, which have been exacerbated by the pandemic. In a statement Monday, Save the Children warned of "a 'second wave' of dropouts as returning students who have fallen behind in their learning fear they have no chance of catching up."

To tackle the potential crisis in Uganda, Save the Children has launched "Catch-up Clubs," which assess children and teaches them at the required level to help them regain literacy and other learning, with child protection support and cash assistance for families struggling to send them to school.

"As schools begin to reopen across the country, it is critical that all girls and boys have access to the support they need to successfully return to the classroom," Edison Nsubuga, head of education at Save the Children in Uganda, said in a statement Monday. "Many children have fallen behind in school as a result of the COVID-19 pandemic. Children who are behind in their learning are less likely to unlock their potential as adults. However, when children receive the learning boost they need and have access to quality education, they can reach their full potential."

The United Kingdom has launched a new advertising campaign that urges pregnant women who have not yet received a COVID-19 vaccine or booster shot to do so as soon as possible.

Testimonies of pregnant women who have had the shots will be played out in ads across U.K. radio stations and on social media starting Monday. The new campaign urges pregnant women "don't wait to take the vaccine" and highlights the risks of COVID-19 to both mother and baby as well as the benefits of getting vaccinated, according to a press release from the U.K. Department of Health and Social Care.

The press release cited the latest data from the U.K. Health Security Agency that suggests COVID-19 vaccination is safe for pregnant women and provides strong protection against the virus for both mother and baby. The press release also cited data from the U.K. Obstetric Surveillance System that shows more than 96% of pregnant women hospitalized with COVID-19 symptoms between May and October 2021 were unvaccinated, and a third of them required respiratory support. Around one in five women who are hospitalized with COVID-19 need to be delivered preterm to help them recover, and one in five of their babies need care in the neonatal unit.

"Getting a COVID-19 vaccine is one of the most important things a pregnant woman can do this year to keep herself and her baby as safe from this virus as possible," Lucy Chappell, chief scientific adviser to the U.K. Department of Health and Social Care, said in a statement Monday. "We have extensive evidence now to show that the vaccines are safe and that the risks posed by COVID-19 are far greater."

Spain has reported more COVID-19 reinfections in a recent span of two weeks than it has during the rest of the pandemic, according to the latest data from a Spanish public health research institute.

Data from the Carlos III Health Institute shows there were 20,890 repeat infections reported in Spain from Dec. 22, 2021, to Jan. 5, 2022. That figure tops the 17,140 reinfections reported in the European country from the start of the pandemic in March 2020 to Dec. 22, 2021. The data includes confirmed, probable and possible cases.

Alfredo Corell, an immunology professor at Spain's University of Valladolid, told Spanish news site NIUS that the rising number of reinfections were due to the new and highly transmissible omicron variant.

"Prior to this variant, reinfections were anecdotal at the global level," Corell told NIUS. "Omicron has changed this paradigm."

In southern Africa, where the variant was first identified in November, preliminary research suggests that omicron is three times more likely to cause reinfections compared to other known variants of the virus, including the highly contagious delta. However, symptoms of reinfected individuals appear to be mild, according to Anne von Gottberg, a microbiologist at South Africa's National Institute for Communicable Diseases who is studying omicron.

"Previous infection used to protect against Delta," von Gottberg said during a press briefing on Dec. 2. "But now, with Omicron, that doesn't seem to be the case."