WASHINGTON - A blizzard of questions from bewildered patients, friends and even other doctors have inundated Maryland primary care doctor Kavita Patel in the last few days, since the Biden administration authorized a fourth dose of coronavirus vaccine for millions of Americans 50 and older. 

Some sought advice on whether to try to time their fourth shots to the next wave of infections. Others were frantic about getting a dose immediately. A few younger than 50 but with chronic health problem wondered why they had been left out.

But this time around, the federal government offered little detailed guidance about who should get the additional dose and when. Instead, the Centers for Disease Control and Prevention said Tuesday that immunocompromised and older Americans could get the fourth shot while stopping short of recommending they do so - a change from some previous decisions regarding coronavirus vaccines. CDC Director Rochelle Walensky said those most likely to benefit fall into two categories: those 65 and older, or anyone 50 and older with underlying medical conditions who are at higher risk for severe disease from COVID-19.

"It does feel like we're a little bit of an afterthought," Patel, a primary care doctor at Mary's Center, a community health center, said of physicians, noting she received no outreach from the CDC or other federal agencies. "We're understaffed, under resourced, and we're being asked to exert clinical nuance when we don't have the tools to do it."

Some doctors and experts shared that view, arguing the agency's scant guidance left them responsible for complicated clinical decisions in a landscape with unclear and constantly evolving data. They also said federal agencies offered little warning that new guidelines regarding second boosters were coming, leaving them unprepared and confused by the policy shift.

Others said they agreed with the decision by the Food and Drug Administration and CDC, saying federal officials could not risk delaying second boosters for those who are most vulnerable since the omicron subvariant BA. 2, which is already fueling infection spikes in Europe and Asia, may cause a surge here. Those experts noted that older Americans are more likely to have weaker immune systems and people over 50 are more likely to have underlying health risks.

They also said risk varies widely from person to person, and doctors and patients should have the opportunity to make individual choices about fourth shots based on a slew of factors, including their age, health, the level of infection where they live and the number of months since their last shot.

The new guidelines come amid a series of confusing and conflicting messages on booster shots since last summer, when federal officials and outside experts fiercely debated whether they were needed. As the omicron wave began, federal officials strongly recommended them and relied on boosters as a critical tool to protect people.

But booster uptake has been lower than desired, including among older Americans at increased risk for severe disease. Doctors said they're still struggling to get people their first booster, let alone beginning discussions about a second.

"There's been unfortunately a string of guidance and statements made by various departments and agencies, FDA and CDC included, throughout the pandemic and most of them have not been very clear or easily interpretable," said James Lawler, an infectious-disease physician and co-director of the Global Center for Health Security at the University of Nebraska Medical Center. "And this one is the same."

"There needs to be a lot more guidance put out for physicians and providers as well. It's a complicated landscape. There's lots of data, the data are potentially all over the map and it's very hard even for infectious-disease doctors to keep up with all of the latest data," Lawler said.

Adding to the confusion was the CDC's new guidance on booster shots for those who received the Johnson & Johnson vaccine. The agency released data showing that the nearly 17 million Americans who received the one-shot coronavirus vaccine are less protected against serious illness and hospitalizations than those who got the Pfizer-BioNTech and Moderna shots. It greenlighted a second booster, either of the Pfizer-BioNTech or Moderna shots, for the 1.3 million people who received Johnson & Johnson's vaccine and a Johnson & Johnson booster at least four months ago.

For Patel, who works in a clinic that treats mostly Medicaid patients, many of whom received the Johnson & Johnson vaccine, that guidance was especially confounding. The CDC's language was so confusing, she said, that it was unclear which Johnson & Johnson recipients should be getting mRNA boosters and how many shots those patients should be getting.

"The Johnson & Johnson language was utterly confusing," Patel said. "I had to call my friends and I'm like, explain this to me."

Some experts said the agencies made the best call they could against the backdrop of limited data, much of it from Israel, an evolving virus and the steady growth of BA. 2 cases in the United States.

"I think this is the right move because there's minimal downside to getting a booster and they appear to be safe," said Albert Ko, an infectious-disease physician and epidemiologist at the Yale School of Public Health. "A lot of the risk is going to have to be decided by individual people given the evidence gap and uncertainty. And having individuals make that decision at this point is the right way to go."

The question of additional boosters four months after the last dose had sparked days of discussion among administration health officials in the Biden administration and extensive debate within the wider scientific community. In addition to scant data, administration officials grappled with uncertainty about whether the highly transmissible omicron subvariant BA. 2 poses a major threat and a political environment in which much of the American population is ambivalent - or, in some cases, hostile - toward vaccines and public health recommendations.

While data shows the vaccines are safe and that there is little downside to getting another shot, Biden officials did not feel the data was strong enough to tell all eligible individuals they should get a second booster shot.

Outside experts have debated the age at which second boosters should be made available - and the broader question about whether and when all eligible adults will need a fourth dose of the mRNA vaccines. That will be discussed April 6 by the FDA's outside vaccine experts, who will also consider whether the vaccines should be adjusted to combat potential variants more effectively.

Many scientists who support a fourth dose for some older adults see little evidence that people under 60 or even 65 would clearly benefit. Federal officials said they were expanding eligibility to a lower age to avoid confusion and because underlying health risks were relatively common among people older than 50.

"I don't think we have good clarity," said Jason Goldman, a physician in Florida who is also a liaison member of the CDC's panel of vaccine experts, the Advisory Committee on Immunization Practices.

"I would have liked to have seen more clarification," Goldman said " ... [the CDC] could say, 'we don't have the data to give a definitive recommendation, but you can consider it in these scenarios."

Like other clinicians, Goldman said he has been bombarded with questions from colleagues and patients seeking more information. One colleague wanted to know what to advise a healthy patient in his 60s, who has been fully vaccinated and boosted and also been infected with COVID-19. Another asked about a patient in his 80s who had no risk factors.

Goldman said he has passed on questions to CDC staff and hopes the agency can provide more clarity by updating its website in coming days.

At the same time, he said evidence in support of a fourth dose is limited and mixed, based largely on data from Israel. But it's hard to compare the United States to Israel, Goldman said, because of that nation's high vaccination rate and comparative homogeneity.

Doctors say one of their biggest challenges in trying to help their patients make decisions on fourth shots will be time. They typically have around 15 minutes with each patient, and it's impractical to expect they'll be able to walk through a patient's medical history and determine whether the fourth shot makes sense for them.

Guidance can't cover every individual case, but if that guidance clearly lays out ... these are the things you should consider, that makes it easier for a primary care physician to make these decisions," Lawler said. "They don't have three hours with every patient to dig through the