“These asymptomatic people may mount an immune response to infection before experiencing symptoms, or they may tolerate infection without symptoms,” explained Ryan. “While it is not entirely clear why some people do not develop symptoms and some people develop only mild symptoms, we know some factors that increase risk of serious infection.”

For example, she said older age is an important risk factor for serious COVID-19 infection. “There is no absolute age when risk increases, but 80% of COVID-19 deaths have occurred in people older than age 65.”

Many underlying medical conditions can increase the risk of serious COVID-19, including but not limited to chronic lung, heart, and kidney disease; diabetes; cancer; and immune-compromising conditions, such as obesity and pregnancy. Smoking can also increase the risk for illness, she said.

New strains

Over the last weeks of 2020 and first weeks of 2021, new variants of SARS-CoV-2 have been identified across the globe, alarming the public and raising concerns about the efficacy of the vaccines.

“Coronaviruses are part of a large family of viruses that are capable of genetically evolving or changing over time,” explained Ryan. “In fact, SARS-CoV-2 likely arose from a major genetic change in a coronavirus that was first identified in 2019.”

According to Cowden, “genetic substitutions in the virus genome will lead to the emergence of new strains as SARS-CoV-2 continues to circulate in the human population throughout the world.” Some of these new strains, or mutations, may lead to increased or decreased “fitness” of the virus, that is, “the changes in how easily it transmits or in virulence — how sick it makes people. 

However, although some of these variants appear to be easily transmitted, they cause the same spectrum of symptoms, Ryan explained. “Spike-protein-based immune responses seem to be similar in new variants, which means current vaccines should be effective against these variants.”

Nonetheless, she said, scientists are examining the evolution of virus variants very carefully. Recent news reports from the United Kingdom say the new variant they identified among their population may not only be more contagious but could be more deadly as well.

How vaccines work

Pharmaceutical companies, universities, and government organizations around the world have been working against and around the clock since the beginning of the COVID-19 pandemic to develop a safe and effective vaccine to counter the rates of contagion. Pfizer-BioNTech and Moderna received the FDA’s emergency use authorization in the United States in mid-December 2020 for people ages 16 and older and 18 and older, respectively, but other promising candidates are in development by additional professional teams, including Janssen, AstraZeneca, and Novavax.

“The different vaccine candidates are all targeting the SARS-CoV-2 spike protein,” said Cowden. They work by presenting an antigen — a substance that the body does not recognize — to the body, she said.

The experts explained that antigens are introduced as a small piece of coding material, called messenger RNA (mRNA), into a few immune cells. It is encased in a lipid coat that allows it to enter only the outer part of the cells. It does not integrate into the person’s DNA or replicate, and it breaks down quickly, so it’s only present in the cell briefly.

“That mRNA then tells the cells to express ‘spike protein,’ resulting in the immune system making antibodies to destroy the spike protein,” said Ryan, adding that mRNA vaccines must be handled very carefully and often maintained at very cold temperatures to work properly.

Vaccines use different platforms to introduce the spike protein to the body to elicit the immune system’s response of blocking or killing the virus if a person becomes infected after vaccination, added Cowden. “Those antibodies then continue to be made by the body and can bind and/or neutralize the virus if an individual becomes infected after vaccination.”

Said Ryan: “The immune system also develops memory immune cells, so that any newly introduced spike protein would also be destroyed. Spike protein is similar to a protein on the coat of coronaviruses, so an immune response to spike protein can equate to immunity from coronavirus (SARS-CoV-2).”

Pfizer partnered with BioNTech to develop an mRNA vaccine that is administered in two injections given at least 21 days apart. Moderna developed a similar mRNA vaccine that also requires administration as two injections given at least 28 days apart. The mRNA technology in these does not include any virus.

Janssen, a subsidiary of Johnson & Johnson, developed a viral-vector vaccine to be administered in one injection. And AstraZeneca, in partnership with the University of Oxford, also developed a viral-vector vaccine that is to be administered in two injections at least 28 days apart. Both pharmaceuticals are likely to submit their products to FDA for emergency use authorization in the coming weeks.

“The Janssen and AstraZeneca/Oxford vaccines are both replication-deficient adenovirus-vectored vaccines,” said Cowden.

This means they use a different virus, such as measles or adenovirus, which is genetically engineered so that it can produce coronavirus proteins in the body and trigger the immune system to make antibodies against those proteins.

“After spike protein is expressed, the immune response is similar to that of mRNA vaccines: The immune system makes antibodies and memory immune cells that target spike protein. This response equates to immunity from SARS-CoV-2,” said Ryan.

The viruses used in viral-vector vaccines are weakened or inactivated, so they cannot cause disease or harm humans, said the experts. Viral-vector vaccines are also relatively fragile and must be maintained at temperatures that allow them to remain intact to work optimally.

Novavax developed an adjuvanted protein subunit vaccine to be administered in two injections at least 21 days apart. The experts explained this type of vaccine usually works by introducing a component of the virus — the antigen — into the body along with an adjuvant, a molecule composed of a cholesterol-like substance that stimulates the immune system, prompting the immune system to make antibodies against the protein antigen, resulting in immunity from the SARS-CoV-2 virus that causes COVID-19.

“Vaccines that use adjuvanted protein subunits introduce the spike protein directly,” said Ryan.

Unlike the others, this type of vaccine remains stable in refrigerated temperatures. When data from ongoing clinical trials is complete, Novavax will likely submit an application to FDA for emergency use authorization.

Why must some vaccines be kept in such cold temperatures?

Each vaccine is made differently. And each company is in different stages of testing the stability of its vaccine at different temperatures over time. However, the storage temperature requirements of each depend on the stability of the materials used to make the vaccine and conformation of the antigen, which in this case, is the spike protein, said Cowden.

She explained the SARS-CoV-2 virus requires a certain configuration of the spike protein so it can bind to ACE2 receptors, a type of enzyme, on human cells. “If the protein in the vaccine changes conformation so that that the [binding domain] RBD is not exposed, then the antibodies the body makes against the vaccine antigen won’t be targeted against it,” said Cowden. This may result in them not blocking the virus as well.

Why are we required to get two doses of some vaccines?

While single doses of vaccines prompt an immune response, Ryan explained that some vaccines must be taken in two doses to ensure a stronger and more long-lasting response in more people. In other words, while a primary shot will provide immunity, the second shot will boost it and build on the immunity from the first dose to generate a more durable response.

“When scientists begin to develop a vaccine, they look for immune responses in laboratory studies and initial small studies in humans,” said Ryan. “The scientists then select vaccine doses and timing that are most promising, and these vaccine schedules are used in large clinical trials.” 

In the case of COVID-19 vaccines, she said there is only strong information available on the doses and timing used in the large trials, for which most products required two-dose schedules.

How do we know vaccines are safe? 

According to Ryan, new medications and vaccines undergo rigorous safety reviews before they are approved for use in the United States. In fact, she said, initial studies must demonstrate safety before they demonstrate effectiveness.

“Safety continues to be evaluated closely in all phases of study and even after a new vaccine is approved for general use,” she said. “The manufacturers and federal authorities, including the Centers for the Disease Control and Prevention, FDA, and DOD, are evaluating the experiences of vaccine recipients very closely.”

Another factor to consider is the commitment from regulatory groups like FDA, said Cowden. “They have strict standards and regulations that must be followed to approve a vaccine or an antibody.”

In fact, she added, the FDA made its standards tougher for COVID vaccines by setting minimum requirements for the effectiveness of products submitted for approval. “By doing this, the FDA has confirmed their strong commitment to only approving a vaccine that could offer immunity to the majority of the population.”

In the case of COVID-19 vaccines that were recently authorized for use, more than 10,000 people received these products in studies, and more than 22 million doses had been administered in the United States under emergency use authorization as of Jan. 24, said Ryan. And although they are known to be reactogenic, or capable of causing a reaction, the side effects they cause — sore arms, headaches, fever — generally last no more than 1-3 days and can be treated with over-the-counter medicines, like ibuprofen or acetaminophen.

“All immediate allergic reactions have been successfully treated at the vaccination clinic sites,” she added. “No safety concerns have substantially changed the recommendation for the general public to receive these vaccines.”

Serious adverse reactions are much rarer but tend to get much more media coverage, said Ryan. “Serious allergic reactions have occurred in about 1 in 100,000 people immediately after vaccine administration.”

Still, “COVID-19 vaccines are among the most effective vaccines ever produced,” assured Ryan. “The safety of these vaccines will continue to be followed very closely, but people should feel confident in receiving these products.”