(The Hill) – Parents of children under five have faced months of waiting for a coronavirus vaccine, with the occasional moments of hope getting snatched away.

But there are some signs of progress on a COVID-19 vaccine for the youngest children, the only age group where the vaccine is not currently available.  

Here are five things to know: 

Pfizer expects results in April 

Pfizer’s vaccine for children under five has been on a roller coaster. It appeared in February that the vaccine could be moving towards authorization, with a Food and Drug Administration (FDA) advisory panel meeting scheduled. But that was delayed in a surprise move as the company said it would wait for data on a third dose, which is expected to have higher effectiveness.  

Those third dose data could come soon, helping clear the way for potential authorization. Pfizer has previously said the data would come in “early April.” 

CEO Albert Bourla seemed to confirm that timeline this month, when he detailed the company’s decision to wait for data from a third dose.  

“We decided to, after a lot of back and forth, to wait for the third dose so that we can have a very clear picture, because it is equally important also to make sure that the data are believable by the public,” he said on CNBC. “So we waited to see what the full picture will tell and those [data] are coming, it is weeks away.” 

Moderna is in the hunt too  

While much of the attention was previously on Pfizer, Moderna made news this week when it said it would soon be applying, in the “coming weeks” for authorization for its two-dose vaccine for children under six. 

In a positive sign, the company reported that the vaccine was able to produce an immune response similar to what is seen in adults.  

However, the effectiveness of the vaccine in preventing any infection was fairly low, at about 44 percent for children six month to two years, and 37 percent for children two to five.  

Still, experts and the company defended the results, saying those numbers are only for preventing any infection at all, including mild illness, and the numbers for the most important goal, preventing severe disease, are likely much better. That has been the case under omicron for adult vaccines, which have shown to be much more effective at preventing severe disease than in preventing any infection at all.  

“The efficacy against infection was 44 percent,” Anthony Fauci, the White House chief medical advisor, said of the Moderna results for children this week. “Now, that may seem like a low number, but in the era of Omicron, this is actually quite comparable to the efficacy against infection in what we’re seeing now with other vaccines.” 

The two companies are taking different strategies 

Once Pfizer’s data comes in April, the FDA will be able to compare the two-dose Moderna vaccine against the three-dose Pfizer vaccine.  

Each dose of Moderna’s is bigger, though, which could aid the effectiveness of its two doses. The Moderna vaccine for children is 25 micrograms, a quarter the size of the adult dose, while the Pfizer vaccine for children is three micrograms, just one-tenth the dose for adults.  

William Moss, a vaccine expert at Johns Hopkins University, said the companies were trying to strike the right balance on the size of the dose, enough to be effective but not so big that it causes more side effects.

“They’re trying to find this balance between what’s sufficient amount of vaccine dosage to protect the children, weighing these transient side effects like fever or soreness,” Moss said.  

“The Pfizer- BioNTech vaccine may have gone a little bit too low on their dosage, and so in order to improve the vaccine efficacy, they’re going with a three-dose,” he added.  

The vaccines are showing good results on safety 

There have not been red flags on safety on the children’s vaccines.  

Moderna reported this week that its trials showed a “favorable safety profile” that was generally similar to the adult vaccines.   

A small percentage reported fevers after vaccination, and a very small percentage, 0.2 percent, reported fevers over 104 degrees.  

There were no deaths or cases of myocarditis, which is inflammation of the heart.  

“The safety profile was quite good — nothing more or less than you would naturally see with so many other vaccinations,” Fauci said this week of the Moderna results. 

There is enough supply, but there’s also hesitancy 

Amid a funding battle in Congress, there is concern that the government will not have enough money to buy fourth doses for all American adults if those are required. But there is enough supply for doses for young children, once the vaccine is authorized. 

“We do have that supply secured for kids under the age of six, so that is good news,” White House COVID-19 response coordinator Jeff Zients said this week.

“And we’ll have those vaccinations available at tens of thousands of locations across the country that parents know and trust,” he said.  

Vaccination rates have been lagging among the group of children where the vaccine is already available, though, with only about a quarter of children 5-11 fully vaccinated.

So while some parents are anxiously awaiting the vaccine for young kids, others will be in no hurry.

“That’s our least vaccinated age group,” Moss said of children 5-11. “I anticipate that coverage will even be lower among children younger than 5 years of age when vaccines become available.”