一年多前，当第一批冠状病毒疫苗运往全国各地时，数百万美国人急切地等待轮到他们注射疫苗，希望这将导致恢复正常。

2021年春天，在每个成年人都有资格接种疫苗后，每天有超过200万人接种第一剂疫苗。然而，最近几个月，随着大多数愿意接种疫苗的人现在都接种了疫苗，接种率直线下降。

接受第一次新冠肺炎疫苗接种的美国人数量目前处于疫情低点，每天开始接种的美国人不到8万人。此外，自去年12月以来，接受加强注射的人数也大幅下降，从每天注射100万次加强注射降至不到14万次。

“当地疫苗授权的取消和omicron激增的结束可能会导致首次疫苗和加强剂的停滞，”波士顿儿童医院流行病学家兼美国广播公司新闻撰稿人约翰·布朗斯坦博士说。“不幸的是，在这一点上，工具箱中没有多少工具可以鼓励人们跟上时代。”

尽管在美国，未接种疫苗的人肯定比接种疫苗的人少，但仍有数千万美国人未接种疫苗和未接种疫苗。在全国范围内，超过5800万符合条件的美国人仍未接种疫苗，而8760万美国人----约占目前符合加强接种条件的一半----尚未接受补充剂量。

在兴趣下降的情况下，一些科学家和卫生官员表示，美国人可能需要在今年秋天增加一种疫苗，或在未来增加季节性疫苗，以应对疫苗免疫力下降或新型冠状病毒变异。

白宫首席医疗顾问安东尼·福奇博士在上个月的一次新闻发布会上说:“我们正在非常仔细地实时监控未来对额外加强或第四次注射mRNA或第三次注射J&J的潜在要求，如果需要，我们将根据数据的发展更新建议。”。

接受美国广播公司采访的专家们关注的是如何在保持信任的同时传达这一信息，以及额外的剂量如何可能进一步加剧全国范围内获取和护理的不平等。

自秋季以来，免疫缺陷的美国人已经可以选择接受第四剂mRNA。然而，对于一般公众来说，额外剂量的益处仍然不清楚。

在上周接受彭博电视台采访时，福奇表示，疾病控制和预防中心正在“逐月”研究数据，如果持续率继续下降，官员们将不得不决定是否开始提供第四剂，特别是对那些高危人群，如老年人。

然而，即使有数据显示需要第四次注射，说服美国人再次注射可能会带来一系列新的挑战。

塔夫茨医疗中心的传染病医生和医院流行病学家Shira Doron博士告诉美国广播公司新闻说:“我认为我们可以预期看到第四剂的摄入比第三剂少。”"围绕疫苗接种计划的目标，改变信息传递会有很大帮助."

她说，有些人错误地将突破性感染的增加作为不接种疫苗的理由。

因此，专家说，公共卫生专家强调疫苗接种的好处，以及如何显著降低感染严重疾病或死亡的风险是很重要的。

去年12月，在奥米克隆占主导地位的时期，未接种疫苗的人死于新冠肺炎病毒的可能性是接种一针强生疫苗或两针辉瑞和Moderna疫苗的人的14倍。

卫生专家还警告说，如果最终需要第四剂，卫生官员概述增加剂量的目的将是很重要的。

“我们是在努力预防所有的感染，还是在努力预防严重的疾病?”多伦问。“公开信息诚实地表明对感染的有效性正在减弱，并强调只有当对严重疾病的有效性已经减弱时才需要增加剂量，并且只针对已经发生这种情况的人群，这可能有助于恢复信任和增加疫苗的摄入。”

布朗斯坦补充说，显示预防严重疾病和死亡的证据最终将是最重要的。

“临床数据，结合真实的证据，必须表明额外的注射对严重疾病和死亡提供了关键的保护，”他说。

专家们特别关注的是，在当前的强化免疫运动中，种族差异不断扩大，以及这种摄入差距可能对已经面临严重新冠肺炎后果风险增加的人群产生的影响。

黑人和棕色人种目前在促进努力方面落后，只有39.5%的合格西班牙裔/拉美裔美国人得到促进，43.8%的合格黑人美国人得到促进。亚裔美国人领先于所有种族/族裔群体，58.9%的合格人口得到了提高。

布朗斯坦解释说:“疫苗的推出凸显了在获取和教育方面的严重不平等，导致了不同种族和族裔之间疫苗接种率的差异。”

如果将来需要更多的疫苗，专家们担心不平等会加深。

“第四枪战略很可能只会进一步加剧保护方面的不平等，除非伴随着直接努力，使整个人口跟上时代，”布朗斯坦补充说。

随着卫生官员对未来几个月和几年的计划，多伦建议有几种潜在的方式来进行，包括在新冠肺炎出现季节性模式时改用年度疫苗接种，或者等待有效性不断减弱的迹象，然后建议第四剂疫苗，特别是对那些高危人群。

“任何需要额外加强剂量的决定都将基于该机构可获得的数据，”FDA的一名代表在一份声明中告诉ABC新闻。

无论官员们决定如何推进，专家们表示，让美国人相信疫苗的重要性和好处是至关重要的，因此，低新冠肺炎疫苗接种率不仅会破坏复苏前景，还可能导致新变种出现时的另一次感染激增。

布朗斯坦说:“如果我们希望美国公众支持，第四次尝试的理由必须令人难以置信地令人信服。”。“重点应该继续放在初级保健提供者和一线卫生保健工作者身上，继续教育公众疫苗的价值。”

With millions still unboosted against COVID, public health experts face tricky messaging around potential 4th shot

When the first coronavirus vaccines were shipped out across the country more than a year ago, millions of Americans waited eagerly for their turn to get a shot, hoping that it would lead to a return to normal.

In the spring of 2021, after every adult became eligible for the vaccine, over 2 million people a day were getting their first dose. However, in recent months, with most of those willing to get their shots now inoculated, vaccination rates have plummeted.

The number of Americans who are receiving their first COVID-19 vaccine now stands at a pandemic low, with fewer than 80,000 Americans initiating vaccination each day. Further, since December, the rate of people getting boosted has also fallen significantly, dropping from 1 million booster shots administered a day to less than 140,000.

"Dropping of local vaccine mandates and the end of the omicron surge are likely contributing to a stalling out in first time vaccines and boosters," said John Brownstein, Ph.D., an epidemiologist at Boston Children's Hospital and an ABC News contributor. "Unfortunately, at this point, there are not many tools left in the toolbox to encourage people to be up to date."

Although there are certainly fewer unvaccinated than vaccinated people in the U.S., tens of millions of Americans remain unvaccinated and unboosted. Across the country, more than 58 million eligible Americans remain unvaccinated, while 87.6 million Americans -- about half of those currently eligible to be boosted -- have yet to receive their supplemental dose.

Amid the declining interest, some scientists and health officials say it is possible Americans could need an additional booster this fall, or seasonal boosters in the future, to address waning vaccine immunity or new coronavirus variants.

"The potential future requirement for an additional boost or a fourth shot for mRNA or a third shot for J&J is being very carefully monitored in real time, and recommendations, if needed, will be updated according to the data as it evolves," White House chief medical adviser Dr. Anthony Fauci during a press briefing last month.

The experts interviewed by ABC News are concerned about how to convey that message while maintaining trust as well as how additional doses might further exacerbate inequities in access and care around the country.

Since the fall, immunocompromised Americans have already had the option to receive a fourth mRNA dose. However, for the general public, the benefit of additional doses still is not clear.

In an interview with Bloomberg TV last week, Fauci said that the Centers for Disease Control and Prevention is studying data on a "month-by-month basis," and should durability rates continue to fall, officials will have to decide whether to begin offering a fourth dose, particularly to those at higher risk, such as the elderly.

However, even if data emerges indicating the need for a fourth dose, convincing Americans to get another shot may present a new set of challenges.

"I think we can expect to see less uptake of fourth doses than we saw of third doses," Dr. Shira Doron, an infectious disease physician and hospital epidemiologist at Tufts Medical Center, told ABC News. "A change in the messaging around the goal of the vaccination program would help a lot."

Some people point erroneously to the increase in breakthrough infections as a reason to not get vaccinated, she said.

Thus, experts say, it is important for public health experts to emphasize the benefit of vaccination, and how dramatically reducing the risk of developing severe illness or dying if infected.

In December, a period of omicron dominance, unvaccinated people were 14 times more likely to die of COVID-19 compared to people who received the one-shot Johnson & Johnson shot or two shots of Pfizer and Moderna vaccines.

Health experts also caution that if a fourth dose is eventually needed, it will be important for health officials to outline what the purpose of an additional dose would be.

"Are we trying to prevent all infections, or are we trying to prevent severe disease?" Doron asked. "Public messaging that is honest about the waning effectiveness for infection and focuses on a need for additional doses only when effectiveness against severe disease has waned, and only for those populations in whom that has happened, might help restore trust and increase vaccine uptake."

Evidence exhibiting protection against severe illness and death, will ultimately be paramount, Brownstein added.

"Clinical data, combined with real-word evidence, must show that additional shots provide critical protection against severe illness and death," he said.

Francine Orr/Los Angeles Times via Getty Images, FILE

In this Jan. 29, 2022, file photo, a woman receives a COVID-19 vaccine at the historic Fir...

Experts are particularly concerned about the continued growing racial disparity in the current booster drive, and the impact such gaps in uptake could have on populations that are already at increased risk of severe COVID-19 outcomes.

Black and brown Americans are currently lagging in the booster effort, with only 39.5% of eligible Hispanic/Latino Americans boosted, and 43.8% of eligible Black Americans boosted. Asian Americans lead every race/ethnicity group, with 58.9% of the eligible population boosted.

"Vaccine rollouts have highlighted critical inequities in access and education, leading to concerning differences in vaccination rates across race and ethnicity," Brownstein explained.

If additional shots are needed in the future, experts worry about deepening inequities.

"A fourth shot strategy is likely to only further inequities in protection unless accompanied with direct efforts to bring the entire population up to date," Brownstein added.

As health officials plan for the months and years ahead, Doron suggested there are several potential ways to proceed, including switching to an annual vaccination should a seasonal pattern with COVID-19 emerge or waiting for continued signs of waning effectiveness and recommending a fourth dose then, particularly to those at high risk.

"Any determination that additional booster doses are needed will be based on data available to the agency," a representative from the FDA told ABC News in a statement.

Regardless of how officials decide to move forward, experts say it will be essential to convince Americans of the importance and benefits of vaccines, and thus, that low COVID-19 vaccination rates could not only undermine recovery prospects, but potentially also lead to another surge of infections in the advent of a new variant.

"The case for a fourth shot needs to be incredibly compelling, if we expect the American public to get on board," Brownstein said. "The focus should continue to be on primary care providers and frontline health care workers to continue to educate the public on the value of vaccines."