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It's been around a year and a half since the COVID-19 was first reported. The cases of COVID-19 increase across the globe, so is the panic among researchers to develop an effective vaccine to control the spread of the infection.
Vaccines like Covaxin, Pfizer, Covishield, Moderna and Sputnik for COVID-19 were manufactured by different manufacturers at a rapid pace and were approved for emergency use in various countries. These vaccines were made using different strategies such as live-attenuated virus, protein subunit, viral vector and nucleic. Almost all the vaccines which have been approved for use probably have two doses given around 4-12 weeks
The question which is raised related to vaccine doses is whether it is safe to get vaccinated from two different vaccine manufacturers.
Some research says that mixing and matching doses of COVID-19 approved vaccines from two different manufacturers, or vaccines that are made with two different strategies, can help provide long-lasting immunity, while other studies say it may cause certain serious side effects. Let's get to the details of it.
What Is Com-COV Study And How Does It Work?
The Com-COV study is a study based on comparing COVID-19 vaccine schedule combinations. The study is raising questions on the increase or decrease of vaccine effectiveness after mixing and matching them.
At present, there are a number of coronavirus vaccines available in various countries, which use two doses: ‘prime' first dose and ‘boost' second dose given a few weeks later.
The vaccines which are used in the study are AstraZeneca and Pfizer. AstraZeneca is made from a live-attenuated adenovirus from chimpanzees while Pfizer is made from mRNA of the COVID-19 spike protein.
This trial study is to find out the response of the immune system when they are primed with one vaccine type and boosted with another after a period of time. This mixing and matching of two different vaccines can help create a more flexible immunisation programme, thus allowing people to get vaccinated quickly (in case there's a shortage of vaccine) and give information on extending the gap between prime and boost.
According to the CDC, the efficacy of mixed COVID-19 vaccines have not been evaluated yet while the WHO says there isn't enough data available on the efficacy of the combination vaccines.
As more new COVID-19 vaccines will become available, more vaccines will be included in the trial. 
Note: The vaccine based on Oxford's study on COVID-19 is manufactured by AstraZeneca, a UK-based pharmaceutical company. Covishield is the local name of the Oxford-AstraZeneca vaccine, which is manufactured by the Serum Institute in India.
Could Mixing COVID-19 Vaccines Boost Immune Response?
According to a study conducted on around 663 people, mixing and matching of AstraZeneca and Pfizer can help boost the immune response. 
In the study, the participants who were enrolled were already vaccinated with the first dose of Oxford-AstraZeneca vaccine. Two-third of the participants among them was randomly picked to receive the second dose of Pfizer-BioNTech, at least eight weeks after the first dose.
The results show that Pfizer has boosted the immune system of the AstraZeneca-dosed participants as their bodies have developed higher levels of antibodies, compared to people who received the same dose of either of the vaccines.
What Researchers Say
Zhou Xing, an immunologist at McMaster University in Hamilton (Canada) said that Pfizer boost is even stronger than the two doses of Covishield, however, the results of the comparison are not clear between people receiving mixed doses of AstraZeneca and Pfizer and those receiving both Pfizer vaccines.
Another London-based immunologist Daniel Altmann said that the mixing of vaccines makes sense in situations, when people will need the third dose to prolong immunity against COVID-19 emerging variants. 
He added that repeated doses of the same vaccines might be ineffective as the immune system will develop immunity only against the vaccine administered. So, with mixed doses, the immune system may develop a range of immunity, including immunity against the COVID-19 variants.
However, according to Dr VK Paul, Member (Health) of the NITI Aayog, the mixing and matching of two COVID-19 vaccines is scientifically and theoretically possible, but due to the lack of scientific evidence, no such recommendations could be made. 
Are There Any Side Effects?
To date, the mix and match of vaccines have caused higher rates of fever, compared to people who received two doses of the same vaccine. Fever is considered a common side effect of probably all vaccines and could be treated easily with medications.. 
Other similar symptoms which were observed are chills, fatigue, malaise, joint pain, headache and muscle ache.
As the trial on using two different vaccine doses is still under consideration, some vaccine types such as Sputnik V, which is developed by the Gamaleya Research Institute (Russia) using two different vectors for both the doses could be set as an example.
The first vector Ad26 is developed by Johnson & Johnson while the second vector Ad5 is developed by CanSinoBio, a Chinese company. A Lancet study has shown that Sputnik V provides consistent strong protective effects for people of all age groups and could demonstrate stronger immunity compared to other vaccines.
This shows that vaccines made with two different vectors could help build stronger immunity, compared to vaccines made with the same vector for both doses. 
The result of the Com-COV trial mentioned above is projected to be available in June 2021. However, the trial reports so far say that the combination of Covishield and Pfizer can be safe and effective in controlling the spread, compared to two doses of the same vaccine.
In the future, more research on the mixing and matching of approved COVID-19 vaccines will be carried out to effectively dealing with the emerging COVID-19 variants.