COVID-19病毒不同种类疫苗的差异

2021
03/02

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桑葛石
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作者:露丝·杰森·希克曼,医学博士;

医学评审人:Anju Goel,医学博士,公共卫生硕士

更新时间:2021年3月1日

桑葛石(译)

在COVID-19新冠状病毒首次出现后不久,科学家们就开始研发疫苗,以防止感染的进一步传播。这是一项艰巨的任务,最初研究人员对病毒知之甚少,能否研发出疫苗也是未知之数。后来科研人员取得了前所未有的进展,设计了多种疫苗,而且比以往其它疫苗的研发速度快,取得了惊人的进步。全球范围内不同的商业化和非商业化团队团队使用了一些或相似或不同的方法来解决疫苗研发问题。

普通疫苗研发过程

疫苗的研发需要一系列的程序来确保最终产品的安全性有效性。首先,动物的基础研究和临床前研究的阶段;第二,疫苗进入第一阶段研究,重点是测试安全性;第三,进如第二阶段研究,重点是测试有效性;第四,进行三期试验,研究数万名患者的有效性和安全性。如果在这一点上情况仍然良好,疫苗可以提交FDA审查。

在应对COVID-19时,最先是由CDC(疾病防治中心)在专门的紧急授权状态下发布疫苗。也就是说疫苗还没有做FDA(食品及药物管理局)所要求的进一步试验研究就分发给了一部分公众,不过FDA和CDC也将继续监测任何意外的安全问题。

我们关心的是,截止目前,哪些疫苗可用,谁能使用这些疫苗,以及安全性如何。

COVID-19疫苗

目前,美国已经有三种疫苗获批上市,获批疫苗生厂商分别为辉瑞、Moderana和强生公司。

截至2020年12月初,全球已有50多种不同的疫苗进入人类临床试验。不过更多的的疫苗仍处于临床前研究阶段,即动物研究或其他实验研究。在美国,另外三种COVID-19疫苗目前处于三期临床试验的某个阶段。如果它们显示出有效性和安全性,更多正在研发的疫苗可能最终获批。

即使COVID-19疫苗经过FDA授权获批的上市,并不意味着每个人都能马上能注射,因为产能还不够,在医疗保健部门工作的人和需要长期护理的居民可能会被优先注射。随着产能的增加,更多的安全性和有效性的信息能被掌握,也将有更多的人能够注射这些疫苗。

疫苗一般如何发挥功效?

所有疫苗的设计目标都有一定相似性,疫苗是为了帮助人们对COVID-19的病毒产生免疫力,以防止人们出现新冠病毒症状。注射疫苗后,如果人们暴露在病毒中,其感染病毒的机会将大大降低。

免疫系统激活

免疫系统是包含一系列复杂细胞的系统,用于识别和消除体内的传染性生物,如病毒。为了设计有效的疫苗,研究人员放大了人体免疫系统的潜在力量,用许多不同的复杂方式做到这一点。

为了消灭病毒,人体有着很多复杂的生理机制,其中,被称为T细胞和B细胞的免疫细胞起着重要作用。T细胞识别病毒上的特定蛋白质并结合它们,最终杀死病毒;B细胞在制造抗体方面起着至关重要的作用,小蛋白也能中和病毒并帮助破坏病毒。如果身体遇到一种新的病毒,这些细胞需要一段时间才能学会识别它们,这就是人在第一次生病后需要一段时间才能康复的原因之一。

T细胞和B细胞在长期保护性免疫中也起着重要作用。再次感染后,某些长寿命的T细胞和B细胞会立即启动识别病毒上的特定蛋白质,如果它们看到这些相同的病毒蛋白,它们会在人体有机会生病之前杀死病毒,可能人们还会生病,但不像第一次被感染时那么严重。

疫苗激活长期免疫

疫苗可以帮助人体发展长期保护性免疫,而不必首先经历主动感染。疫苗使人体的免疫系统暴露在某种抗原下,帮助免疫系统制造对抗这些抗原的T细胞和B细胞,这种抗原可能是某种病毒,例如:COVID-19病毒。由于人体在疫苗下激活了这些细胞,如果人体将来暴露在病毒中,这些细胞就会立即杀灭病毒。因此,人体不太可能有严重的感染症状,或者可能根本没有任何症状。

COVID-19疫苗如何与免疫系统相互作用以获得这种保护性免疫方面存在差异,正在研发的COVID-19疫苗可分为两大类:

经典疫苗:包括活(弱)病毒疫苗、灭活病毒疫苗和基于蛋白质的亚基疫苗。

新型疫苗:包括基于核酸的疫苗(如基于mRNA的疫苗)和病毒载体疫苗。

经典的疫苗方法已经被用来制造市场上几乎所有的人类疫苗。从2020年12月开始在美国进行三期试验的五种COVID-19疫苗中,除一种用的经典方法以外,其它疫苗都是基于新方法。

活病毒疫苗是使用一种仍然活跃和存活的病毒来引起免疫反应。病毒已经被改变和严重削弱,以至于它几乎没有引起任何症状,如麻疹、腮腺炎和风疹疫苗。由于它们仍然有活病毒,这些类型的疫苗需要更广泛的安全测试,与其他方法相比,它们可能更容易引起重大不良事件。对于免疫系统受损的人来说,这种疫苗可能是不安全的,此外,活病毒疫苗需要小心保存让疫苗处于有效状态。但活病毒疫苗的优点是会引起持续很长时间的强烈的免疫反应,通常一针就可以达到免疫的效果,这些疫苗也不需要使用额外的佐剂。

灭活疫苗是最早研制的普通疫苗之一,将灭活的病毒注入体内,它不能真正感染人体,但免疫系统仍然被激活,并触发长期免疫记忆,帮助保护人体,如果小儿麻痹症疫苗。使用灭活病毒的疫苗需要注射多剂。它们不会引起像活疫苗那样强烈的免疫反应,也比活病毒疫苗更安全、更稳定。灭活病毒疫苗和弱病毒疫苗需要专门的安全协议和完善的产品研发和生产模式。

基于蛋白质的亚基疫苗

该类疫苗也是传统疫苗,虽然可能使用了一些新的制备方法。基于蛋白质的亚基疫苗不使用灭活或减弱的病毒,而是使用病原体的一部分来诱导免疫反应。科学家们仔细地选择了病毒中最能使免疫系统运转的一小部分。对于COVID-19,可能使用到其一个蛋白质到一组蛋白质用于诱导免疫反应。有时,从活病毒中纯化,能找到一种特定的蛋白质,被认为是免疫系统的良好触发因素。有时,科学家也会选择合成一种与病毒蛋白质几乎相同的蛋白质,即重组蛋白,如有些乙肝疫苗由这种特定类型的蛋白质亚基制成的,还有人类乳头瘤病毒(HPV)的疫苗。 这些疫苗不能引起任何活性感染,它们只含有一种病毒蛋白或一组蛋白质,而不是病毒复制所需的完整病毒机制。蛋白质亚基疫苗的优点是它们比使用整个病毒的疫苗产生的副作用更少。例如,20世纪40年代第一批针对百日咳的疫苗使用的是灭活菌,后来的百日咳疫苗采用了蛋白质亚基的方法,消除了明显的副作用。蛋白质亚基疫苗的另一个优点是,它们的存在时间比新型疫苗技术长。蛋白质亚基疫苗也要缺点:蛋白质亚基疫苗需要使用佐剂来促进免疫反应,这可能有其潜在的不良影响;与使用活病毒的疫苗相比,免疫力可能不够持久;与使用新技术的疫苗相比,研发时间更长。

核酸疫苗

新型疫苗技术是围绕核酸构建的:DNA和mRNA。DNA是人体从父母那里继承的遗传物质,而mRNA是细胞用来制造蛋白质的遗传物质的一种拷贝。

核酸疫苗来自实验室人工合成的一小部分病毒的mRNA或DNA,这些mRNA或DNA最终用于触发免疫反应,这种遗传物质包含所需特定病毒蛋白的编码。疫苗通过使用特定的载体分子,遗传物质进入人体的细胞,然后人体细胞使用这些遗传信息来产生抗体。DNA和mRNA疫苗可以制造非常稳定安全的疫苗,也能产生强烈和持久的免疫反应。与DNA疫苗相比,mRNA疫苗可能具有更大的安全性。用DNA疫苗理论上有可能部分DNA会插入到人的DNA中,虽然通常不会产生这个问题,但在某些情况下,突变的风险可能导致癌症或其他健康问题,基于mRNA的疫苗并不构成这种理论风险。

在过去的几年里,研究人员研究了许多不同的基于mRNA的疫苗,用于艾滋病毒、狂犬病、寨卡病毒和流感等传染病。然而这些疫苗都没有达到试验终点,基于DNA的疫苗也是如此,尽管其中一些已被批准用于兽医用途。 

辉瑞和Moderna的COVID-19疫苗都是基于mRNA的疫苗,其他几种基于DNA和mRNA的疫苗目前正在全世界进行临床试验。

病毒载体疫苗

病毒载体疫苗与基于mRNA或DNA的这些疫苗有很多相似之处,它们只是用一种不同的方式将病毒遗传物质进入人体的细胞。病毒载体疫苗使用的是另一种病毒的一部分,这种病毒经过基因改造后不具有传染性,这种病毒特别擅长进入人体细胞。如在灭活病毒(如腺病毒)的帮助下,编码COVID-19尖峰蛋白的特定遗传物质被带入细胞,就像其他类型的mRNA和DNA疫苗一样,细胞本身产生的蛋白质会触发免疫反应。从技术的角度来看,这些疫苗可以被分离成病毒载体,可以继续在体内复制和不能复制的病毒载体,但这两种情况下的原则都是一样的。

与基于mRNA的新方法相比,研究人员在病毒载体疫苗方面有更多的经验。如这种方法已经被安全地用于埃博拉疫苗,并对其他病毒(如艾滋病毒)的疫苗进行了研究。其中一个优点是,与其他新的疫苗技术相比,生产单一疫苗免疫方法更容易,它也可能更容易适应世界各地许多不同设施的大规模生产。

阿斯利康强生制药公司的COVID-19疫苗是基于一种非复制病毒载体。

人们需要不同的COVID-19疫苗吗?

人们希望能够提供多种安全有效的疫苗,任何单一制造商都不可能迅速释放足够的疫苗来服务于全世界的人口。如果生产出几种不同的安全和有效的疫苗,进行广泛的疫苗接种就会容易得多。

此外,不同的疫苗具有不同的特性,以满足不同的需要。有些需要一定的储存条件,如冷冻;有些需要不具备的高科技设施中生产;有些疫苗可能提供更持久的免疫力;对某些人群,如老年人和免疫系统有问题的人,活病毒疫苗可能不会被建议使用。

然而,我们现在没有足够的数据来正确地比较这些疫苗的有效性,随着时间的推移,信息将变得更加清晰。一旦有多个疫苗可用,将是尽可能多的人接种疫苗的关键。只有通过这些努力,才能真正结束这一流行病。

原文文末附原文链接

Types of COVID-19 Vaccines How They Work: Differences and Similarities

By Ruth Jessen Hickman, MD

Medically reviewed by Anju Goel, MD, MPH

Updated on March 1, 2021  

Very soon after the first appearance of the new coronavirus (SARS-CoV-2) that causes COVID-19, scientists began working to develop vaccines to prevent the spread of infection and end the pandemic. This was a huge task, because little was known about the virus initially, and at first it wasn’t even clear if a vaccine would be possible.

Since that time, researchers have made unprecedented strides, designing multiple vaccines that may ultimately be utilized on a much faster timeframe than has ever been done for any previous vaccine. Many different commercial and non-commercial teams over the world have used some overlapping and some distinct methods to approach the problem.

General Vaccine Development Process

Vaccine development proceeds in a careful series of steps, to make sure the final product is both safe and effective. First comes the phase of basic research and preclinical studies in animals. After that, vaccines enter small phase 1 studies, with a focus on safety, and then larger phase 2 studies, with a focus on effectiveness.

Then come much larger phase 3 trials, which study tens of thousands of patients for both effectiveness and safety. If things still look good at that point, a vaccine can be submitted to the FDA for review and potential release.

In the case of COVID-19, the CDC is first releasing qualifying vaccines under a specialized Emergency Use Authorization status. That means they will be available to some members of the public even though they haven’t received as extensive study as is required for a standard FDA approval.

Even after the release of vaccines under Emergency Use Authorization, the FDA and CDC will continue to monitor for any unexpected safety concerns.

COVID-19 Vaccines: Stay up to date on which vaccines are available, who can get them, and how safe they are.

COVID-19 Vaccine Update

A COVID-19 vaccine developed by Pfizer was granted an Emergency Use Authorization on December 11, 2020, based on data from its phase 3 trials. As of mid-December 2020, it is the only vaccine to achieve this.

A vaccine sponsored by Moderna has also submitted to the FDA for Emergency Use Authorization, based on data of effectiveness and safety in their phase 3 trial. AstraZeneca has also submitted preliminary information on their COVID-19 vaccine to the FDA based on data from phase 3 trials.

As of early December 2020, more than 50 different vaccines worldwide have moved into clinical trials in human beings. Even more vaccines are still in the preclinical phase of development (in animal studies and other laboratory research).

In the U.S., three additional COVID-19 vaccines are currently in some stage of phase 3 trials or will enter very soon. Several other phase 3 trials are ongoing worldwide. If they demonstrate effectiveness and safety, more of the vaccines under development may ultimately be released.

Even though a COVID-19 vaccine has been released by the FDA, not everyone will be able to get it right away, because there won’t be enough. Priority will go to certain people, like people who work in healthcare and residents of long-term care facilities.

As more vaccines become available and even more information about safety and efficacy become known, more people will be able to get these vaccines.

How Do Vaccines Work Generally? 

All the vaccines designed to target the new coronavirus disease share some similarities. All are made to help people develop immunity to the virus that causes the symptoms of COVID-19. That way, if a person is exposed to the virus in the future, they will have a greatly reduced chance of getting sick.

Immune System Activation

To design effective vaccines, researchers leverage the natural powers of the body’s immune system. The immune system is a complex array of cells and systems that work to identify and eliminate infectious organisms (such as viruses) in the body.

It does this in a lot of different complex ways, but specific immune cells called T cells and B cells play an important role. T cells identify specific proteins on the virus, bind them, and ultimately kill the virus. B cells perform critical roles in making antibodies, small proteins that also neutralize the virus and help make sure it is destroyed.

If the body is encountering a new type of infection, it takes a while for these cells to learn to identify their target. That’s one reason it takes you a while to get better after you first become sick.

T cells and B cells also both play an important role in long-term protective immunity. After an infection, certain long-lived T cells and B cells become primed to recognize specific proteins on the virus right away.

This time, if they see these same viral proteins, they get right to work. They kill the virus and shut down the reinfection before you ever have a chance to get sick. Or, in some cases, you might get a little bit sick, but not nearly as ill as you did the first time you were infected.

Activation of Long-term Immunity by Vaccines

Vaccines, such as those designed to prevent COVID-19, help your body develop long-term protective immunity without having to go through an active infection first. The vaccine exposes your immune system to something that helps it develop these special T cells and B cells that can recognize and target the virus—in this case the virus that causes COVID-19.

That way, if you are exposed to the virus in the future, these cells will target the virus right away. Because of this, you’d be much less likely to have severe symptoms of COVID-19, and you might not get any symptoms at all. These COVID-19 vaccines differ in how they interact with the immune system to get this protective immunity going.

The vaccines under development for COVID-19 can be broken up into two overarching categories:

Classical vaccines: These include live (weakened) virus vaccines, inactivated virus vaccines, and protein-based subunit vaccines.

Next-generation vaccine platforms: These include nucleic acid-based vaccines (such as those based on mRNA) and viral vector vaccines.

Classic vaccine methods have been used to make almost all the vaccines for human beings currently on the market. Of the five COVID-19 vaccines that have begun phase 3 trials in the U.S. as of December 2020 (or which will start very soon), all but one are based on these newer methods.

Live (Weakened) Virus Vaccines

These vaccines are a classic type.

How They Are Made

A live virus vaccine uses a virus that is still active and alive to provoke an immune response. However, the virus has been altered and severely weakened so that it causes few, if any symptoms. An example of a live, weakened virus vaccine that many people are familiar with is the measles, mumps, and rubella vaccine (MMR), given in childhood.

Advantages and Disadvantages 

Because they still have live virus, these types of vaccines require more extensive safety testing, and they may be more likely to cause significant adverse events compared to those made by other methods.

Such vaccines may not be safe for people who are people who have impaired immune systems, either from taking certain medications or because they have certain medical conditions. They also need careful storage to stay viable.

However, one advantage of live virus vaccines is that they tend to provoke a very strong immune response that lasts a long time. It’s easier to design a one-shot vaccine using a live virus vaccine than with some other vaccine types.

These vaccines are also less likely to require the use of an additional adjuvant—an agent that improves the immune response (but which may also have its own risk of side effects).

Inactivated Virus Vaccines

These are also classic vaccines.

How They Are Made

Inactivated vaccines were one of the first kinds of general vaccines to be created.They are made by killing the virus (or other type of pathogen, like a bacteria). Then, the dead, inactivated virus is injected into the body.

Because the virus is dead, it can’t really infect you, even if you are someone that has an underlying problem with your immune system. But the immune system still gets activated and triggers the long-term immunological memory that helps protect you if you’re ever exposed in the future. An example of an inactivated vaccine in the U.S. is the one used against polio virus.

Advantages and Disadvantages 

Vaccines using inactivated viruses usually require multiple doses. They may also not provoke quite as strong a response as a live vaccine, and they may require repeat booster doses over time. They are also safer and more stable to work with than with live viruses vaccines.

However, working with both inactivated virus vaccines and weakened virus vaccines requires specialized safety protocols. But they both have well-established pathways for product development and manufacturing.

COVID-19 Vaccines in Development

No vaccines undergoing clinical trials in the U.S. are using either live virus or inactivated virus approaches. However, there are several phase 3 trials taking place abroad (in China and India) that are developing inactivated virus vaccine approaches, and at least one vaccine is being developed utilizing a live vaccine method.6

Protein-Based Subunit Vaccines

These are also a classical type of vaccine, although there have been some newer innovations within this category.

How They Are Made 

Instead of using inactivated or weakened virus, these vaccines use a part of a pathogen to induce an immune response.

Scientists carefully select a small part of the virus that will best get the immune system going. For COVID-19, this means a protein or a group of proteins. There are many different types of subunit vaccines, but all of them use this same principle.

Sometimes a specific protein, one that is thought to be a good trigger for the immune system, is purified from live virus. Other times, scientists synthesize the protein themselves (to one that is almost identical to a viral protein).

This lab synthesized protein is called a “recombinant” protein. For example, the hepatitis B vaccine is made from this type of specific type of protein subunit vaccine.

You might also hear about other specific types of protein subunit vaccines such as ones based on virus-like particles (VLPs). These include multiple structural proteins from the virus, but none of the virus’ genetic material. An example of this type of vaccine is the one used to prevent human papillomavirus (HPV).

For COVID-19, almost all the vaccines are targeting a specific viral protein called the spike protein, one which seems to trigger a strong immune response. When the immune system encounters the spike protein, it responds like it would as if it were seeing the virus itself.

These vaccines can’t cause any active infection, because they only contain a viral protein or group of proteins, not the full viral machinery needed for a virus to replicate.

The different versions of the flu vaccine provide a good example of the different types of classical vaccines available. Versions of it are available made from live virus and from inactivated virus. Also, protein subunit versions of the vaccine are available, both ones made from purified protein and ones made from recombinant protein.

All these flu vaccines have slightly different properties in terms of their effectiveness, safety, route of administration, and their requirements for manufacturing.

Advantages and Disadvantages

One of the advantages of protein subunit vaccines is that they tend to cause fewer side effects than those that use whole virus (as in weakened or inactivated virus vaccines).

For example, the first vaccines made against pertussis in the 1940s used inactivated bacteria. Later pertussis vaccines used a sub-unit approach and were much less likely to cause significant side effects.

Another advantage of the protein subunit vaccines is that they have been around longer than newer vaccine technologies. This means that their safety is better established overall.

However, protein subunit vaccines require the use of adjuvant to boost the immune response, which can have its own potential adverse effects. And their immunity may not be as long-lasting compared to vaccines that use the whole virus. Also, they may take longer to develop than vaccines using newer technologies.

Vaccines in Development for COVID-19

The Norovax COVID-19 vaccine is a type of subunit vaccine (made from a recombinant protein) expected to begin phase 3 clinical trials in the U.S. in December 2020.15 Others may enter phase 3 trials in 2021.

Nucleic-Acid Based Vaccines

The newer vaccine technologies are built around nucleic acids: DNA and mRNA. DNA is the genetic material you inherit from your parents, and mRNA is a kind of copy of that genetic material that is used by your cell to make proteins.

How They Are Made

These vaccines utilize a small section of mRNA or DNA synthesized in a lab to ultimately trigger an immune response. This genetic material contains the code for the specific viral protein needed (in this case, the COVID-19 spike protein).

The genetic material goes inside the body’s own cells (by using specific carrier molecules that are also a part of the vaccine). Then the person’s cells use this genetic information to produce the actual protein.

This approach sounds a lot scarier than it is. Your own cells will be used to produce a type of protein normally made by the virus. But a virus needs a lot more than that to work. There’s no possibility of being infected and getting sick.

Some of your cells will just make a little COVID-19 spike protein (in addition to the many other proteins your body needs daily). That will activate your immune system to start forming a protective immune response. 

Advantages and Disadvantages

DNA and mRNA vaccines can make very stable vaccines that are very safe for manufacturers to handle. They also have the good potential to make very safe vaccines that also give a strong and long-lasting immune response.

Compared to DNA vaccines, mRNA vaccines may have an even greater safety profile. With DNA vaccines, there is the theoretical possibility that part of the DNA might insert itself into the person’s own DNA. This usually wouldn’t be a problem, but in some cases there is a theoretical risk of a mutation that might lead to cancer or other health issues. However, mRNA-based vaccines don’t pose that theoretical risk.

In terms of manufacturing, because these are newer technologies, some parts of the world may not have the capacity to produce these vaccines. However, in places where they are available, these technologies have the capacity for much more rapid vaccine production than earlier methods.

It’s partly due to the availability of these techniques that scientists have been hopeful about producing a successful COVID-19 vaccine so much more quickly than has been done in the past.

Vaccines in Development for COVID-19

Researchers have been interested in DNA and mRNA-based vaccines for many years. Over the past several years, researchers have worked on many different mRNA-based vaccines for infectious diseases like HIV, rabies, Zika, and influenza.

However, none of these other vaccines have reached the stage of development leading to official approval by the FDA for use in humans. The same is true of DNA-based vaccines, although some of these have been approved for veterinary uses.

Both the Pfizer and Moderna COVID-19 vaccines are mRNA-based vaccines. Several other DNA and mRNA-based vaccines are currently undergoing clinical trials around the world.

Viral Vector Vaccines

Viral vector vaccines have a lot of similarity to these vaccines based on mRNA or DNA. They just use a different mode of getting the viral genetic material into a person's cells.

Viral vector vaccines use part of a different virus, one that has been genetically modified to not be infectious. Viruses are particularly good at getting into cells.

With the help of an inactivated virus (such as an adenovirus) the specific genetic material encoding the COVID-19 spike protein is brought into the cells. Just as for other types of mRNA and DNA vaccines, the cell itself produces the protein that will trigger the immune response.

From a technical standpoint, these vaccines can be separated into viral vectors that can continue to make copies of themselves in the body (replicating viral vectors) and those that can't (non-replicating viral vectors). But the principle is the same in either case.

Just like other types of nucleic acid-based vaccines, you can’t get COVID-19 itself from getting such a vaccine. The genetic code only contains information to make a single COVID-19 protein, one to prompt your immune system but which won’t make you sick.

Advantages and Disadvantages

Researchers have a little more experience with viral vector vaccines compared to new approaches such as those based on mRNA. For example, this method has been safely used for a vaccine for Ebola, and it’s undergone study for vaccines for other viruses such as HIV.However, it’s currently not licensed for any applications for humans in the U.S.

One advantage of this method is that it may be easier to produce a single shot method for immunization in contrast to other new vaccine technologies. Compared to other newer vaccine techniques, it also may be easier to adapt for mass production at many different facilities around the world.

Vaccines in Development for COVID-19

The AstraZeneca vaccine is based on a non-replicating viral vector. Janssen pharmaceutical company has also developed a COVID-19 vaccine based on a non-replicating viral vector that is currently in phase 3 trials. (It is the only one currently undergoing phase 3 trials in the U.S. that is a one-shot method).

Do We Need Different COVID-19 Vaccines?

Ultimately, it’s hoped that multiple safe, effective vaccines will become available. Part of the reason for this is that it will be impossible for any single manufacturer to quickly release enough vaccine to serve the population of the whole world. It will be much easier to perform widespread vaccination if several different safe and effective vaccines are produced.

Also, not all these vaccines will have exactly the same properties. Hopefully, multiple successful vaccines will be produced that might help meet different needs.

Some require certain storage conditions, like deep freezing. Some need to be produced in very high-tech facilities that aren’t available in all parts of the world, but others use older techniques that can be more easily reproduced. And some will be more expensive than others.

Some vaccines may turn out to provide longer-lasting immunity compared to some others, but that isn’t clear at this time. Some might turn out to be better for certain populations of people, like the elderly or people with certain medical conditions. For example, live virus vaccines will probably not be advised for anyone who has problems with their immune system.

However, we don’t have enough data, now, to properly compare these vaccines in terms of their effectiveness (and hopefully minimal safety issues). That will become clearer with time. 

Once one or more vaccines are available, it will be key for as many people as possible to get vaccinated. Only through such efforts will we really be able to end the pandemic. 

本文由作者自行上传,并且作者对本文图文涉及知识产权负全部责任。如有侵权请及时联系(邮箱:nanxingjun@hmkx.cn
关键词:
DNA,疫苗,病毒,差异,细胞

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