CNN Philippines (Metro Manila) — COVID-19 vaccines developed by American pharmaceutical giant Pfizer and its German partner BioNTech, British-Swedish company AstraZeneca, China-based Sinovac, and most recently Russian group Gamaleya Research Institute of Epidemiology and Microbiology obtained emergency use approval from the Food and Drug Administration.

The FDA is still reviewing the applications for emergency use of the coronavirus shot made by India-based Bharat Biotech.

An emergency use authorization is issued for unregistered drugs and vaccines amid a public health emergency, a state which the country is in due to the COVID-19 pandemic. The EUA, which hastens approval for vaccines that normally takes years, is only valid for the duration of the public health emergency declaration, or upon the issuance of a certificate of product registration or CPR. CPR is necessary to sell the drugs and vaccines to the general public.

Meanwhile, three pharmaceutical firms are authorized to conduct local clinical trials for their COVID-19 vaccines. They are Johnson & Johnson unit Janssen Pharmaceuticals, China-based biotechnology company Clover Biopharmaceuticals and Sinovac.

Vaccines are designed to teach the body how to defend itself from the virus, and the serious diseases it causes. But how are they different from each other?

Here’s a look at the types of COVID-19 vaccines that will be tested and used locally.

Inactivated vaccine

The Sinovac shot, which was the first to be rolled out in the country, is called an inactivated vaccine. This vaccine type uses a killed form of SARS-CoV-2, one of the oldest and tried-and-tested methods of prompting an antibody response. Bharat Biotech's COVAXIN, developed with the Indian Council of Medical Research, as well as the vaccine made by China's Sinopharm, work in the same way.

In early March, Bharat Biotech announced the COVAXIN demonstrated 81% efficacy in preventing COVID-19 after two doses, which were given 28 days apart. The company’s Phase 3 trial enrolled 25,800 volunteers, including those above 60 and people with comorbidities. Meanwhile, Sinopharm earlier announced its vaccine -- which had been administered to President Rodrigo Duterte's security detail and allegedly to other high-ranking government officials despite the absence of regulatory approval -- had a high efficacy rate among the Asian ethnicity group -- at over 79%.

While deemed safe, an inactivated vaccine may not always generate a strong immune response, so a number of doses may be required to realize its benefit, according to the World Health Organization.

The regulator recommended giving Sinovac's CoronaVac to clinically healthy individuals aged 18 to 59. FDA Director General Eric Domingo earlier said CoronaVac is not recommended for medical workers exposed to COVID-19 patients, elderlies, and those with co-morbidities because it showed a low efficacy rate of 50.4%, in late-stage trials in Brazil. But Health spokesperson Maria Rosario Vergeire later said that while clinical trials show lower efficacy in preventing mild symptoms, CoronaVac is still 100% effective against moderate and severe symptoms.

No adverse reactions are so far linked to the Sinovac vaccine, according to the World Allergy Organization. Side effects are usually mild, experts said.

The Chinese government donated to the Philippines 600,000 doses of CoronaVac, 167,798 of which were already injected to healthcare workers, government data as of March 16 showed.

Viral vector vaccine

Unlike the inactivated vaccine, this type of shot utilizes a different virus as a vector, which delivers instructions to the cells to produce the COVID-19 spike protein, which is needed to trigger an immune response. The virus has been genetically modified so as not to cause an illness in humans.

Viruses are made up of a core of genetic material — either DNA or RNA — covered in a coat of proteins called capsid. SARS-CoV-2, the virus that causes COVID-19, uses RNA and its surface is called the spike protein.

Experts have given a word of warning about the vector vaccine: It may be less effective in individuals who had preexisting immunity to the modified virus used as vector. This is because “humans develop immune responses when exposed to viruses,” according to the Centers for Disease and Control Prevention, a health agency based in the United States.

The vaccine of AstraZeneca, which is developed with a team at Britain’s University of Oxford, is an example of a vector vaccine, with a weakened virus as the critical component. Makers of this vaccine use an adenovirus, the virus that causes common cold, to carry the genetic material needed in viral protein building.

The country has received over 525,600 doses of AstraZeneca vaccines through the global initiative COVAX. These shots are for health care workers. WHO country representative Rabindra Abeyasinghe said the country will receive 4.5 million more doses of AstraZeneca vaccines by April or May.

An analysis of phase 3 clinical trials that have not yet undergone peer review showed that the Oxford-AstraZeneca vaccine had an efficacy of 82.4% when two doses were given at least 12 weeks apart.

Likewise, Gamaleya’s Sputnik V — which is 91.6% effective based on clinical trials involving nearly 20,000 people — is a vector vaccine. It uses common cold viruses adenovirus 5 and adenovirus 26.

Janssen’s single-dose vaccine is also a vector vaccine, which like Gamaleya, uses an adenovirus 26. The Johnson & Johnson vaccine was 66% effective at preventing moderate to severe COVID-19, 28 days after the vaccination, according to the company. 

mRNA vaccine

One particular vaccine type is getting a lot of attention, and for the right reasons — the messenger ribonucleic acid or mRNA shot. It is a new type of vaccine, but it took researchers decades to develop the technology behind it, which means it can be trusted, experts said. Unlike conventional vaccines, mRNA vaccine does not use an actual virus, just a genetic sequence of the virus-causing disease. No mRNA vaccine has been authorized for use in any disease, but COVID-19 has turned the tide.

This type of coronavirus shot works by injecting mRNA molecules into the body, which the cells read as genetic instructions to make the spike protein. The body will then produce antibodies against the spike protein since the immune system has never seen it before and therefore sees it as an intruder. The main goal of a vaccine is to teach the body what the virus looks like. Once the immune system learns what its appearance is, it will destroy the actual virus if it gets into the body.

The shots made by Pfizer-BioNTech and Moderna are mRNA vaccines, and they have shown high efficacy — well over 90%— in clinical trials involving tens of thousands of participants, with limited side effects. Severe allergies including anaphylaxis have been reported after some received mRNA shots, but these adverse reactions are said to be rare.

Still, health experts are not recommending a second shot to anyone who has had a history of serious allergies to the ingredients (e.g. polyethylene glycol) of the vaccines developed by either Pfizer-BioNTech or Moderna. 

​Subunit vaccine

Another way to make a vaccine is to just use parts of a virus — and this is called the subunit approach. Clover, for example, makes use of the SARS-COV-2's protein coat, with two adjuvants, to provoke antibody production. Adjuvants have been added to vaccine formulations for decades to make the products more effective in fighting off viruses. Clover said in February it is eyeing to begin global phase 2/3 trials in the first quarter of the year to release the interim analysis on vaccine efficacy possibly by the middle of 2021.