CNN  — 

Several more states on Thursday announced Covid-19 vaccines would be available to more people, news that came as the nation’s top infectious disease expert said we might need to inoculate children to reach herd immunity.

But, Dr. Anthony Fauci said people are too focused on the thought of herd immunity – the point at which enough people are protected against the virus to suppress spread – for this novel coronavirus.

“I think we should be careful about wedding ourselves to this concept of herd immunity because we really do not know precisely, for this particular virus, what that is,” Fauci told a Senate hearing.

Fauci, the longtime director of the National Institute of Allergy and Infectious Diseases, said he has been estimating that anywhere between 70% and 85% of the population would need to be vaccinated or otherwise immune to the virus to get to the point of herd immunity.

“We don’t really know what that magical point of herd immunity is, but we do know that if we get the overwhelming population vaccinated, we’re going to be in good shape. We ultimately would like to get and have to get children into that mix,” Fauci said during a hearing of the Senate Health, Education, Labor and Pensions Committee.

He said when high school students are vaccinated, the US might reach herd immunity.

More states are widening the number of people eligible to get vaccinated. New Hampshire Gov. Chris Sununu said the residents 50 and older will be eligible as of Monday and the state plans to expand vaccine access to all citizens 16 and older “in just a matter of weeks.”

Missouri Gov. Mike Parson said beginning March 29 vaccination will be available to adults in sectors essential to economic recovery and starting April 9 all adults can get a shot.

Illinois will expand to all residents 16 and older on April 12, Gov. J.B. Pritzker said.

Kentucky Gov. Andy Beshear said starting Monday providers should include individuals 50 and older.

Expert worries about where case numbers are headed

But even as vaccinations increase, the US may be on the cusp of another Covid-19 case surge, one expert says.

“I think we are going to see a surge in the number of infections,” emergency physician Dr. Leana Wen told CNN on Wednesday night. “I think what helps this time though is that the most vulnerable – particularly nursing home residents, people who are older – are now vaccinated. And so we may prevent a spike in hospitalizations and deaths.”

Health officials have repeatedly warned about a potential fourth surge as state leaders eased restrictions and several lifted mask mandates. The first warning sign came when case numbers, after weeks of steep declines, appeared to level off – with the country still averaging tens of thousands of new cases daily. That kind of plateau previously predicted surges, some experts have said.

An usher holds up a mask-required sign during the men's Big Ten college basketball tournament on March 11 in Indianapolis.

Cases of the worrying variants – notably the highly contagious B.1.1.7 variant – have also climbed and are set to become the dominant strain by the end of March or early April.

Meanwhile, governors and local leaders have eased restrictions on indoor gatherings, citing fewer Covid-19 cases and more vaccinations. And spring break crowds are gathering in Florida and other Gulf Coast states and nationwide air travel numbers are hitting pandemic-era highs.

Now, as the country gets closer to 30 million reported infections, average daily cases counts are more than 10% higher in 17 states this week compared to last week, according to Johns Hopkins University data – with nine of those states seeing a rise of more than 20%.

If Covid-19 cases continue to rise in many states, the mass vaccination of our most vulnerable are likely to limit increases in hospitalizations and deaths. People age 65 and older make up more than 80% of all Covid-19 deaths, according to Centers for Disease Control and Prevention data. Yet 66% of people 65 and older have had at least one vaccine dose, and about 39% are fully vaccinated, which sharply cuts down on the risk of hospitalization and death.

Wen still has her concerns, though.

“I think we’re going to see an increase in the number of infections, but not necessarily an increase in hospitalizations and deaths, which again is a really good thing,” she told CNN on Thursday. “But we also note that many governors are not going to reimpose restrictions unless we see our hospitals becoming overwhelmed.

“So we could see a situation of a lot more infections outpacing the ability of our vaccines to work and people letting down their guard but not having the restrictions in place to curb it. And I fear that we may lose, as a result, this race of variants versus vaccines.”

In schools, 3 feet is the new 6 feet

The CDC is expected to update its physical distancing guidelines for schools from 6 feet to 3 feet on Friday, an administration official confirmed to CNN.

US health officials have pointed to a study published last week that showed Massachusetts schools requiring 3 feet of distance between people had no difference in Covid-19 rates compared to those keeping students 6 feet apart. All staff and students above second grade were still required to wear masks.

The change to 3 feet is key to safely reopening schools because most don’t have the space for 6 feet of distancing with all students present. An analysis of studies on reopenings published last week found that school districts in Indiana, Virginia, and Massachusetts have all adopted a 3-foot standard instead of 6 feet.

Those states “have not seen a surge of cases that you would expect if somehow that protection was less adequate,” education and policy expert John Bailey wrote.

At the Senate hearing Thursday, CDC Director Dr. Rochelle Walensky said that the science on this issue had evolved over time.

“Indeed, because 6 feet has been such a challenge there, science has leaned in and there are now emerging studies on the question between 3 feet and 6 feet,” she told Sen. Susan Collins, a Maine Republican. “This is an urgent issue.”

The World Health Organization has recommended physical distancing in schools of at least one meter, or about 3 feet.

You asked, we answered: Your top questions about Covid-19 and vaccines

Nearly 1 in 8 Americans fully vaccinated

Meanwhile, vaccinations have accelerated as officials race to get as many shots into arms as quickly as possible.

About 75 million Americans have received at least one dose of a Covid-19 vaccine, according to CDC data. And about 41 million people are fully vaccinated, or roughly 12% of the US population.

But challenges – including vaccine hesitancy, disinformation and inequities – remain.

The US should soon have plenty of vaccines on hand and will need to start persuading reluctant people to get vaccinated, a top Health and Human Services Department official said Thursday.

“We will have, within 90 days, in essence quadrupled our vaccine supply,” Dr. David Kessler, Chief Science Officer for COVID Response at HHS, said at the Senate hearing. “I believe that we’re going to be shifting from a supply issue to a demand issue pretty soon.”

In addition, the CDC will soon release more guidance on what people can safely do once they’re fully vaccinated, Walensky said.

“We’re revisiting what we should do regarding travel for those who are vaccinated and that should be coming forward soon. That’s going to likely be the next step in this regard,” Walensky said.

The issue is not what’s safe for those who are vaccinated, but what’s safe for their contacts, she said.

“We are still looking at data regarding whether people who are vaccinated can be asymptomatically infected and potentially transmit to other people,” Walensky said. Doctors note that people who are vaccinated can potentially still breathe in virus and have it living in the nose and throat – and could exhale, cough or sneeze infectious virus onto others.

CNN’s Steve Almasy, Melissa Alonso, Adrienne Broaddus, Chris Boyette, Elizabeth Cohen, Deidre McPhillips, Brandon Miller, Nick Neville, Rebekah Riess and Naomi Thomas contributed to this report.