I got ET 2007 and Covid19 was far away, a lot of people have tried to find the reason for the MPN you have. Try to find some real research into the connection Covid19 ans an MPN.
MPN disease is near always from a long standing mutation in the marrow, usually years before you got the Dx. As you note the abnormality was found before your 1st vax, so the causal relation is not possible. I had an abnormality 8 years before Dx . And your rate of increasing PLT values are commonly seen with MPN around the time of Dx. There has not been any reported connection or process between MPN and any vaccine.
In that physical examination, the abnormal item was bilirubin, which showed a high value of 40, which I think is wrong, if it reaches this value, there will be very obvious symptoms, I think this examination result is wrong. I checked later in the hospital and all the indicators were normal
It remains that the mutation near always originates years before the disease presents, possibly even before birth. You should request a mutation test to know which mutation you have and what your mutation level is.
As Swede says here if you can find credible reports on the subject it would inform the discussion.
In my current condition, the size of my spleen is 11.9cm*4.4cm, which is larger than normal. A tiny nodule was found on both sides of my lungs, and the morphologic analysis of bone marrow cells showed that megakaryocytes were significantly active.
It is important to use a clear definition of terms when looking at a possible diagnosis of a MPN. Thrombocytosis can be defined differently by different labs, but diagnostically it is an elevation in PLT > 450. Thrombocytosis can be either primary (Essential Thrombocythemia-ET) or Secondary (Reactive Thrombocytosis).
Secondary thrombocytosis can be caused by a number of things, including bleeding events, iron deficiency, injury, inflammation, and illness/infections. While not common, immunizations (e.g., COVID) can cause a very brief transient rise in platelet levels. Secondary Thrombocytosis resolves when the underlying condition is resolved. You can experience reactive thrombocytosis when you have a MPN. I have seen elevations of 200+ over my baseline level of thrombocytosis when I had a significant GI inflammation or illness.
ET is not related to the COVID or any other vaccine. It is caused by one of three driver mutations in hematopoietic stem cells (JAK2, CALR, MPL). More rarely, there is ET triple-negative where the ET is present diagnostically without the presence of a canonical driver mutation. The strongest confirmation of ET is though a bone marrow biopsy where the bone marrow morphology can be examined. Splenomegaly can be a feature of ET; however, the spleen size you indicate is within the normal range. Your Hematologist can best assess the issue of any increase in spleen size as it relates to your case. This is something that is a part of routine monitoring for MPNs. The bone marrow morphology that indicates ET is "BM biopsy showing proliferation mainly of the megakaryocyte lineage with increased numbers of enlarged, mature megakaryocytes with hyperlobulated nuclei. No significant left-shift of neutrophil granulopoiesis or erythropoiesis and very rarely minor (grade 1) increase in reticulin fibers." pmc.ncbi.nlm.nih.gov/articl...
As Epguy suggests, you need to know your driver mutation status as well as bone marrow morphology to have the most clear diagnosis. Ideally, you should consult with a MPN Specialist rather than a regular hematologist. MPNs are rare disorders and many doctors, including hematologists, have little experience with them. Here is a list of patient recommended MPN docs just in case you need it. mpnforum.com/tsr-the-list/
The WHO criteria are a good place to help understand the biospy results. In table 1 for ET:
"BM biopsy showing proliferation mainly of the megakaryocyte lineage with increased numbers of enlarged, mature megakaryocytes with hyperlobulated nuclei. No significant left-shift of neutrophil granulopoiesis or erythropoiesis and very rarely minor (grade 1) increase in reticulin fibers"
The "no signif" left shift (not-increased) features are increased in PV as seen in col 1.
Has your Dr discussed a deeper gene study? This will look for which mutation you have (or don't) and other mutations that have prognostic value. In ET Jak2, CALR or MPL can be driving it. Less common, it can be none (triple negative) but the "other mutations" could still show results of interest.
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Is bone marrow biopsy necessary?
Just diagnosed with diastolic heart failure and I am shocked! Could this be MPN induced?
New to the site and diagnosed with PV in February 2018
9 month Besremi update 
I have Jak 2 and ET. My platelet count is 700. The other labs are good. 
